with reference to fig1 a , there is shown a network element architecture suitable for the delivery of emergency services in accordance with an embodiment of the present invention . a packet - switched network 100 , which may or may not be the public internet , comprises a backbone to which users have access via customer premises equipment 102 such as a modem 104 in combination with a residential gateway 106 . in some embodiments , the modem 104 and the residential gateway 106 may be combined into a single unit at the customer premises . a voip customer desirous of obtaining telephony services via the packet - switched network 100 may be provided with a special - purpose voip telephone or device that connects directly to the residential gateway 106 . in another embodiment , and as illustrated in fig1 a , the voip customer utilizes a conventional analog telephone 110 which connects to the residential gateway 106 using an analog terminal adapter ( ata ) 112 . the ata 112 permits the voip customer to re - use conventional telephony hardware in a voip environment , thus obviating the need to purchase and maintain a second telephone strictly for ip telephony purposes . generally speaking , however , it is immaterial to the present invention whether the voip customer uses a special - purpose voip telephone or a conventional analog telephone 110 coupled to an ata 112 . in a typical residential application , the packet - switched network 100 is accessed by the modem 104 in the customer premises equipment 102 upon establishing a connection to a modem 108 belonging to a network service provider , commonly an internet service provider ( isp ). the connection is made via an access infrastructure , examples of which include but are not limited to copper telephone lines ( for an adsl modem ) 104 and coax cable ( for a cable modem 104 ). it should be further understood that the present invention applies to the delivery of emergency services not only in a residential context but in other contexts such as business and corporate applications , where access to the packet - switched network 100 may be provided by a server that in some cases is directly connected to the packet - switched network 100 . a voip service provider maintains a registration entity 190 , which voip customers may access via the packet network 100 . in a non - limiting example of implementation , the registration entity 190 may be embodied as a server having a control entity and an i / o . initially , potential voip customers contact the voip service provider via the registration entity 190 . a given voip customer registers with the registration entity 190 and obtains a voip telephone number n ( hereinafter referred to as a “ directory number ”). the voip customer , identified by the directory number n , can then begin to place calls into ( and receive calls from ) the packet - switched network 100 . the registration entity 190 performs various other functions which will be described in further detail later on . at the edge of the packet - switched network 100 there is provided a network element 114 , which may be referred to as a packet switch or softswitch , and which comprises suitable circuitry , software and / or control logic for providing various communication services to voip customers . examples of such communication services include but are not limited to call waiting , call forwarding , and so on . in addition , the network element 114 comprises suitable circuitry , software and / or control logic for exchanging calls with entities outside the packet - switched network 100 . where a call is placed by a voip customer , there are at least two circumstances that require the call to pass through the network element 114 , namely , ( i ) a call placed to a telephone number that is reachable only via the public switched telephone network ( pstn ) 116 and ( ii ) an emergency call . in the former case , the network element 114 detects when a voip customer in the packet - switched network 100 is attempting to reach a destination that can only be reached via the pstn 116 , in which case the call is routed via a network 124 to one of a plurality of gateways 118 a , 118 b that connect to the pstn 116 . in the latter case , the network element 114 detects when a voip customer in the packet - switched network 100 has dialed ( either explicitly or via a speed dial function or in some other way ) an emergency number such as “ 9 - 1 - 1 ”. in such a case , the call , hereinafter referred to as an emergency call , is routed to one of the gateways 118 a , 118 b , which connect not only to the pstn 116 as described above , but also to a network of dedicated emergency lines and public safety answering points ( psaps ), hereinafter collectively referred to as an e911 network 122 . in addition to the above , when handling an emergency call , the network element 114 comprises circuitry , software and / or control logic suitable for outpulsing a “ routing key ” associated with the directory number of the voip customer having placed the emergency call . the routing key accompanies the emergency call as it is routed by the network element 114 to the appropriate one of the gateways 118 a , 118 b . further detail regarding routing keys and the operation of the network element 114 will be given later on in this description . in a specific example of implementation , the network element 114 is the multimedia communication server 5200 from nortel networks limited , brampton , ontario , canada , although it should be understood that the present invention applies equally to other makes , models and types of packet switches or softswitches that have ( or can be configured to have ) the ability to assign a routing key to a voip customers directory number . as previously mentioned , the network element 114 is connected to the gateways 118 a , 118 b via the network 124 . in some embodiments , the network 124 may be part of the packet - switched network 100 while in other embodiments it may not . in still other embodiments , rather than being connected via the network 124 , the network element 114 may be connected to each gateway 118 a , 118 b by a respective communication link that can be optical fiber , coaxial cable , wireless , free - space optical , etc . it is noted that the network 124 ( or the communication link ( s ), as the case may be ) carries multiple telephone calls simultaneously . in an embodiment of the present invention , emergency calls are treated differently from non - emergency calls and therefore it is envisaged that distinct virtual trunk groups will be established for either type of call ( non - emergency and emergency ), as well as for each of the gateways 118 a , 118 b . in addition to communicating with the network element 114 via the network 124 , the gateways 118 a , 118 b in fig1 a also communicate with components of the pstn 116 and the e911 network 122 . specifically , gateway 118 a is connected to the pstn 116 via a plurality of high - capacity switches 128 a , 128 b and is also connected to the e911 network 122 via a plurality of high - capacity switches 132 a , 132 b . similarly , gateway 118 b is connected to the pstn 116 via a plurality of high - capacity switches 148 a , 148 b and is also connected to the e911 network 122 via a plurality of high - capacity switches 152 a , 152 b . it should be understood that the specific architecture shown in fig1 a is merely for purposes of illustration ; in other architectures that are within the scope of the present invention , there may be more or fewer gateways , and not all gateways need be connected to both the pstn 116 and the e911 network 122 . in a specific example of implementation , each or either of the gateways 118 a , 118 b may be embodied as the communication server 2000 from nortel networks limited , brampton , ontario , canada , although it should be understood that the present invention applies equally to other makes , models and types of gateways . an example of a basic function of the gateways 118 a , 118 b is to allow non - emergency calls originated in the packet - switched network 100 to be completed via the pstn 116 ( which is circuit - switched ) and vice versa . another example of a basic function of the gateways 118 a , 118 b is to take emergency calls originated in the packet - switched network 100 and to route them into the e911 network 122 , which is circuit - switched ( much like the pstn 116 ). continuing with the description of the architecture in fig1 a , switch 132 a is connected via a first portion of the e911 network 122 to a first plurality of psaps , including psap 138 a and psap 138 b , while switch 132 b is connected via a second portion of the e911 network 122 to a second plurality of psaps , including psap 138 c and psap 138 d . similarly , switch 152 a is connected via a third portion of the e911 network 122 to a third plurality of psaps , including psap 138 e and psap 138 f , while switch 152 b is connected via a fourth portion of the e911 network 122 to a fourth plurality of psaps , including psap 138 g and psap 138 h . of course , this distribution of psaps is not to be considered as limiting . each switch routes a received call in accordance with a connection map . for an emergency call received at a given one of the switches 132 a , 132 b , 152 a and 152 b , the call will specify a desired psap to be reached . the identity of the desired psap may be expressed in the form of a “ e911 telephone number ”. thus , for example , switch 132 b will recognize an emergency call that has an associated “ e911 telephone number ” which specifies either psap 138 c or psap 138 d , and will route the emergency call accordingly . the e911 telephone number of may correspond to the telephone number of a specific psap along a dedicated in the e911 network 122 , and is usually held confidential by the local exchange carrier . in addition , switches 132 a , 132 b , 152 a and 152 b may have a further ability to forward an emergency call towards a specialized entity other than the psaps shown in the drawings . the desirability of doing so arises when a trained responder at a psap determines that a special agency ( e . g ., police , fire or ambulance ) may need to be contacted . forwarding of the emergency call may be done in accordance with a forwarding table that maps plural emergency telephone numbers to each directory 11 number . each emergency telephone number mapped to a given directory number is associated with a respective forwarding code that signifies either “ police ”, “ fire ” or “ ambulance ”. during an actual call received at a psap , a particular forwarding code would be applied by a trained responder at the psap in question and sent to switch 132 b . upon receipt of the particular forwarding code , switch 132 b is operative to look up the directory number of the call in question and to forward the emergency call towards the appropriate agency using the emergency telephone number for the forwarding code in question . in practice , the trained responder may enter into a three - way conference before the call forward is complete . it has already been mentioned that pstn calls and emergency calls are received from the network 124 over different virtual trunk groups . this makes it a simple task for a particular one of the gateways 118 a , 118 b to determine towards which network ( i . e ., the pstn 116 or the e911 network 122 ) to direct a given call . however , in the case of an emergency call received by , say , gateway 118 a , there is still a question of whether to route the call towards switch 132 a or towards switch 132 b . to this end , gateway 118 a maintains a connection map 134 a which associates each potential received routing key with one of the switches , either switch 132 a or switch 132 b . in addition , gateway 118 a may convert the routing key into a format more understandable to the switches 132 a , 132 b . one example of a more understandable format is the “ e911 telephone number ” format mentioned above . the e911 telephone number accompanies the emergency call as it is routed by the gateway 118 a to the appropriate one of the switches 132 a , 132 b . in an analogous fashion , gateway 118 b maintains a connection map 134 b and also may convert received routing keys into e911 telephone numbers . further detail regarding e911 telephone numbers and the operation of the gateways 118 a , 118 b will be given later on in this description . switches 132 a , 132 b , 152 a and 152 b currently operate entirely within the circuit - switched domain . however , this does not rule out the possibility of the switches 132 a , 132 b , 152 a and 152 b being retrofitted with the functionality of an ip gateway that would allow an ip connection from the network element 114 directly to the switches 132 a , 132 b , 152 a and 152 b via dedicated virtual trunk groups , thus bypassing the need for gateways 118 a and 118 b in this intermediate position . this possibility is envisaged in fig1 c . it is noted that a set of gateways 198 a , 198 b is still used to connect the network element 114 to the legacy switches 128 a , 128 b , 148 a , 148 b leading to the pstn 116 . returning to fig1 a , the psaps 138 a to 138 h are connected to an ali database 140 . the ali database 140 is a known database that stores street addresses and associated telephone numbers , thus enabling a psap operator to obtain the street address corresponding to a given directory number from which an emergency call has originated . the ali database 140 is connected to a 9 - 1 - 1 database management system ( 9 - 1 - 1 dbms ) 150 , which maintains a mapping of street addresses to “ emergency zones ”, such as a municipality , county or district , for example . the 9 - 1 - 1 dbms 150 is accessible to the registration entity 190 , either by a direct link or via the packet - switched network 100 . the architecture in fig1 a also comprises a street address guide ( sag ) 160 , which is accessed by the registration entity 190 , either by a direct link or via the packet - switched network 100 . the street address guide 160 provides validation of a street address in order to determine whether a particular entry corresponds to a realistic address . in accordance with an embodiment of the present invention , certain steps are performed for each voip customer during a provisioning phase , which occurs before the placement of an emergency call by that voip customer , and is now described with reference to the signal flow diagrams in fig2 a through 2h , which correspond to steps 2 - a through 2 - h . in fact , it may be advantageous to perform the following steps during the same general time frame as when the voip customer obtains his or her directory number n . at step 2 - a , the voip customer provides a service address to the registration entity 190 . the service address , which may differ from the billing address , is typically the geographic location of the voip customer , which may be the civic ( street ) address where the voip customer is located , although it is envisaged that in some embodiments it may be the latitude / longitude of the voip customer or some other form of localization data . the manner in which the voip customer provides the service address to the registration entity 190 is not material to the present invention and may include the usage of the web , email , snail mail , etc . it is noted that step 2 - a may be performed at the same time as when the voip customer is first assigned a directory number n and in fact it is envisaged that the execution of step 2 - a may even be made a condition for the delivery of voip services . at step 2 - b , the registration entity 190 validates the service address supplied by the user . this can be achieved by running the service address through a street address guide ( sag ) 160 that is available to the voip service provider . validation provides an assurance that the service address given by the user is a valid address , i . e ., really exists , and therefore will be capable of being meaningfully associated with an emergency zone and its designated psap . if validation at step 2 - b is unsuccessful , then the voip customer may be asked to re - enter the service address with a greater degree of precision or may be prompted to resolve an ambiguity by choosing the service address from a list of two or more address choices . step 2 - b may also be performed interactively with the voip customer and may involve the intervention of a customer service representative . provided validation at step 2 - b is successful , the registration entity 190 proceeds to step 2 - c , which consists of supplying the validated street address to the 9 - 1 - 1 dbms 150 . the 9 - 1 - 1 dbms 150 has the functionality of identifying an emergency zone associated with the service address . in one embodiment , the 9 - 1 - 1 dbms 150 maintains a mapping that associates postal codes ( zip codes ) to emergency zones . thus , a given service address having a given postal code will map to a corresponding emergency zone . at step 2 - d , the 9 - 1 - 1 dbms 150 returns a file processing confirmation 209 to the registration entity 190 . the file processing confirmation 209 may identify the emergency zone ( hereinafter denoted 210 ) associated with the service address in question . at step 2 - e , which may actually be executed before step 2 - d , the 9 - 1 - 1 dbms 150 provides the directory number n and the validated street address to the ali database 140 for storage therein . at step 2 - f , the 9 - 1 - 1 dbms 150 updates the forwarding tables at the switches 132 a , 132 b , 152 a , 152 b , with routing information 202 for the purposes of eventual call transfer to dispatch agencies ( police , fire , ambulance ) as per established routines . in an example , the individual emergency telephone numbers corresponding to police , fire and ambulance agencies which are associated with emergency zone 210 are entered into the forwarding table in association with directory number n . at step 2 - g , which may actually be executed before step 2 - f , the registration entity 190 consults a call routing list ( crl ) 188 , which associates emergency zones 210 to individual “ routing keys ” 214 . the result of step 2 - g is the obtaining of a routing key 214 that corresponds to the emergency zone 210 . by virtue of the association between each directory number n and its emergency zone 210 , and by virtue of the association between each emergency zone 210 and its routing key 214 , it will be apparent that each directory number n will be associated with a routing key 214 . also , since more than one emergency zone may be serviced by the same psap , a plurality of directory numbers n will share the same routing key 214 . at step 2 - h , the registration entity 190 provides the directory number n and the associated routing key 214 ( obtained at step 2 - g ) to the network entity 114 . the network entity 114 enters this information into a table 178 local to the network entity 114 . the table 178 may be stored in the network entity 114 or otherwise directly accessible thereto . fig3 a shows a specific , non - limiting example of the table 178 that is local to the network entity 114 . basically , the table 178 comprises a plurality of records 204 , each containing a directory number n and a related routing key 214 . a particular routing key 214 comprises information that defines a route to be taken by an emergency call in order to reach a particular psap . in one embodiment , not to be considered as limiting , the routing key 214 comprises a gateway identifier 214 a and a routing code 214 b . further detail regarding the purpose and effect of fields 214 a , 214 b will be given later on in this specification . in an alternative embodiment of steps 2 - g and 2 - h , shown in fig2 - i and 2 - j , the registration entity 190 provides the directory number n and the associated emergency zone 210 to the network entity 114 , and it is the network entity 114 that consults a call routing list ( crl ) 188 in order to obtain the appropriate routing key 214 for the emergency zone 210 in question . in this case , and with reference to fig3 b , the table 178 ′ local to the network element 114 would comprise a plurality of records , each containing a directory number n , a related emergency zone 210 and a related routing key 214 . with additional reference now to the diagrams of fig4 - a to 4 - f , placement of an emergency call and operation of the various elements in the architecture of fig1 in a “ call handling ” phase is now described . at step 4 - a , the network element 114 detects an emergency call 400 received from a voip customer associated with a particular directory number n . at step 4 - b , the network element 114 consults the table 178 ( or 178 ′) which is local to the network element 114 and retrieves the routing key 214 for the directory number n . as previously mentioned , the routing key 214 contains a gateway identifier 214 a , which identifies the destination gateway towards which the emergency call 400 should be routed . let this destination gateway be gateway 118 a . in addition , the routing key 214 contains a routing code 214 b which , when interpreted by gateway 118 a , will identify ( i ) a destination switch towards which gateway 118 a should route the emergency call 400 and ( ii ) the destination psap for the emergency call 400 . for the purposes of this example , let the destination switch be switch 132 b and let the destination psap be psap 138 c . at step 4 - c , the network element 114 routes the emergency call 400 onto the virtual trunk group assigned to the destination gateway , in this case gateway 118 a . in addition , as part of step 4 - c , the network element 114 forwards the routing code 213 b along with the emergency call 400 . in an alternative embodiment , the network element 114 forwards the routing key 214 in its entirety . the forwarded information accompanies the emergency call 400 as it is routed to gateway 118 a . at step 4 - d , the gateway 118 a receives the emergency call 400 from the network element 114 . the emergency call 400 is accompanied by at least the routing code 214 b . gateway 118 a reads the routing code 214 b in order to learn ( i ) the identity of the destination switch ( in this case switch 132 b ) towards which the emergency call 400 should be routed by gateway 118 a and ( ii ) the identity of the destination psap ( in this case psap 132 c ) towards which the emergency call 400 should be routed by the destination switch 132 b . additionally , gateway 118 a obtains the e911 telephone number corresponding to the destination psap 138 c , hereinafter denoted 250 . the destination gateway 118 a then proceeds to route the emergency call 400 to the destination switch 132 b and forwards the e911 telephone number 250 along with the emergency call 400 . at step 4 - e , the destination switch ( in this case switch 132 b ) routes the received emergency call 400 . routing is performed on the basis of the e911 telephone number 250 received from gateway 118 a , resulting in the emergency call 400 being transferred onto a dedicated line leading towards the destination psap ( in this case psap 138 c ) over the e911 network 122 . at step 4 - f , once the incoming emergency call 400 is received at the destination psap 138 c , it is handled by a trained responder . with knowledge of the directory number n ( which follows the emergency call 400 from its inception ), the responder obtains the validated service address associated with the directory number n . in one embodiment , the responder queries the ali database 140 upon receipt of the emergency call 400 in order to obtain the validated service address . in an alternative embodiment , the validated service address is pushed by the ali database 140 during a previous step . specifically , after step 4 - e described above , receipt of the emergency call 400 by switch 132 b could be followed by switch 132 b supplying the directory number n to the ali database 140 , which then pushes the validated service address to the destination psap 138 c . in either case , the responder learns the exact geographic location of the caller and can dispatch emergency personnel if necessary . in an example scenario , the responder may determine that a particular type of emergency agency ( police , ambulance , fire ) needs to be dispatched . a forwarding code can be dialed back to switch 132 b from which the emergency call 400 originated . the forwarding code triggers switch 132 b to use its internal forwarding table in order to forward the emergency call 400 to a particular emergency telephone number where the appropriate agency can be reached . since the updating of the forwarding table was done in the provisioning phase at step 2 - f ( as described earlier ), the emergency call 400 will be automatically forwarded to the agency of the appropriate type that is geographically in the best position to handle the emergency call 400 . of course other embodiments of explicit routing using a routing key 214 are within the scope of the present invention . for instance , it is envisaged that the routing code 214 b mentioned above may comprise only the e911 telephone number 250 corresponding to the destination psap . in such a scenario , a gateway that receives the emergency call and the associated e911 telephone number 250 would access a local table to obtain the identity of the switch that is connected to the destination psap . in fact , the functionality of consulting a local table could be relegated to network element 114 , such that it is the network element 114 that determines the ports that need to be used by the gateway when routing the emergency call in question , in order that the call reach the destination psap . fig1 b shows such an embodiment , where the connection maps ( 134 a , 134 b in fig1 a , formerly executed by the gateways 118 a , 118 b , respectively ) have been consolidated into a single connection map executed at the network element 114 . from the above description , it will be noted that the assignment of a routing key 214 to each directory number n permits independence of the directory number n and the destination psap . in other words , there need not be any relationship between the “ area code ” or “ local exchange ” of the directory number n and the destination psap , which is unlike the case with the traditional telephony infrastructure . as a result , voip service providers can assign arbitrary directory numbers to their customers , while ensuring that emergency services will be dispatched effectively by the appropriate psap for each customer . moreover , as has been shown using the example of fig4 - a to 4 - f , emergency calls 400 can be directed to the appropriate psap over a dedicated emergency circuit in the e911 network 122 , rather than over an administrative line , thereby maximally assuring a prompt response by trained personnel . those skilled in the art will appreciate that in some embodiments , the functionality of parts of the network element 114 and / or the registration entity 190 may be implemented as pre - programmed hardware or firmware elements ( e . g ., application specific integrated circuits ( asics ), electrically erasable programmable read - only memories ( eeproms ), etc . ), or other related components . in other embodiments , parts of the network element 114 and / or the registration entity 190 may be implemented as an arithmetic and logic unit ( alu ) having access to a code memory ( not shown ) which stores program instructions for the operation of the alu . the program instructions could be stored on a medium which is fixed , tangible and readable directly by the network element 114 and / or the registration entity 190 , ( e . g ., removable diskette , cd - rom , rom , or fixed disk ), or the program instructions could be stored remotely but transmittable to the network element 114 and / or the registration entity 190 via a modem or other interface device ( e . g ., a communications adapter ) connected to a network over a transmission medium . the transmission medium may be either a tangible medium ( e . g ., optical or analog communications lines ) or a medium implemented using wireless techniques ( e . g ., microwave , infrared or other transmission schemes ). while specific embodiments of the present invention have been described and illustrated , it will be apparent to those skilled in the art that numerous modifications and variations can be made without departing from the scope of the invention as defined in the appended claims .