an illustrative embodiment of the present invention will be described below with reference to the drawings . fig1 is a functional block diagram of an ip telephone set 10 including a call request control device of the present embodiment . as shown in the figure , the ip telephone set 10 is constituted by a base unit 20 and a handset 40 . the base unit 20 is constituted as including an interface circuit 21 , a control unit 22 , a signal processing unit 23 , a codec 24 , a voice amplifier 25 , a voice amplifier 26 , a receiving buffer 27 , a sending buffer 28 , a dial buffer 29 , a hook switch 30 , dial keys 31 , a transmission / redial key 32 , and a lcd display unit 33 . the voice amplifier 26 is connected to a microphone 42 which is incorporated in the handset 40 , and a voice signal which was input to the microphone 42 is amplified in the voice amplifier 26 and is then input to the codec 24 . on the other hand , the voice signal which is output from the codec 24 is amplified in the voice amplifier 25 , whereby the reinforced sound is output through a speaker 41 . the codec 24 includes an a / d converter and d / a converter , in which an analog voice signal which is output from the voice amplifier 26 is converted into a digital voice signal ( voice data ) and supplied to the signal processing circuit 23 , and the voice data which is output from the signal processing circuit 23 is converted into an analog voice signal and output to the voice amplifier 25 . the signal processing circuit 23 is a compressing / expansion circuit for voice data , which is constituted as including a dsp ( digital signal processor ) or the like , in which : voice data supplied from the codec 24 is compressed and stored in the sending buffer 28 ; the compressed voice data which is supplied from the control unit 22 is subjected to expansion processing ; and the expanded voice is supplied to the codec 24 . the interface circuit 21 is a circuit that is constituted as including a mac ( media access control ) circuit and phy ( physical protocol ) circuit , in which : a transmission code sent from a transmission path such as an ip network is subjected to decoding processing and stored in the receiving buffer 27 ; and the voice data supplied from the control unit 22 is encoded and sent to the transmission path . the control unit 22 is a circuit that executes call control , in which : the voice data stored in the sending buffer 28 is read and then packetized into predetermined frames ; the packetized voice data is supplied to the interface circuit 21 ; the voice data is retrieved from the received data accumulated in the receiving buffer 27 and then supplied to the signal processing circuit 23 . the base unit 20 further comprises the dial buffer 29 for accumulating dial data , hook switch ( detection means ) 30 for detecting onhook / offhook status of the handset 40 , dial keys 31 for dial inputs , transmission / redial key 32 for requesting for a call ( including a redial call request ), and lcd display unit 33 for displaying the dial data and the like . fig2 is a plan view of the base unit 20 . as shown in the figure , in addition to the hook switch 30 , dial keys 31 , transmission / redial key 32 , and lcd display unit 33 as described above , the base unit 20 is also equipped with volume controlling keys , a setting key , a cancellation key , a transfer / hook key , a speaker key , a hold key , and the like . the transmission / redial key 32 is a key functioning as a combination of the conventional transmission key and redial key , and can request a call to the dialed number or can redial by performing an input operation of this key . fig3 is a flow chart explaining call request processing executed by the control unit 22 . as shown in the figure , in an offhook state , when a key input is detected ( step s 1 ), the type of the key that was pressed is checked ( step s 2 ). if the pressed key is a dial key , the dial data is stored in the dial buffer 29 ( step s 3 ), whereby the procedure returns to the step s 1 . if the pressed key is the transmission / redial key 32 , it is checked whether there is dial data stored in the dial buffer 29 ( step s 4 ). if there is dial data stored in the dial buffer 29 ( step s 4 ; yes ), the dial data is set as a call destination number ( step s 5 ), and a call request message is sent ( step s 6 ). on the other hand , if dial data is not stored in the dial buffer 29 ( step s 4 ; no ), redial data is set as the call destination number ( step s 7 ), and a call request message is sent ( step s 6 ). preferably , the redial data is of the terminal number in the outgoing to which a call was made most recently , but it is not limited to this . for example , it may be constituted such that a caller can select a terminal number from among the list in the outgoing history or incoming history . if the pressed key is the speaker key , the dial buffer 29 is cleared ( step s 8 ), and the procedure moves on to a standby mode . if the pressed key is the cancellation key , the dial buffer 29 is cleared ( step s 9 ), and the procedure returns to the step s 1 . fig4 is a flow chart explaining call request processing executed by the control unit 22 . as shown in the figure , in an onhook state , when a key input is detected ( step s 10 ), the type of the key that was pressed is checked ( step s 11 ). if the pressed key is a dial key , the dial data is stored in the dial buffer 29 ( step s 12 ), and the procedure returns to the step s 10 . if the pressed key is the transmission / redial key 32 , it is checked whether there is dial data stored in the dial buffer 29 ( step s 13 ). if there is dial data stored in the dial buffer 29 ( step s 13 ; yes ), the dial data is set as a call destination number ( step s 14 ), and a call request message is sent ( step s 15 ). on the other hand , if dial data is not stored in the dial buffer 29 ( step s 13 ; no ), the redial data is set in the dial buffer 29 and further displayed on the lcd display unit 33 ( step s 16 ). the procedure then returns to the step s 10 . with such configuration , when pressing the transmission / redial key 32 twice in the onhook state , by the first input operation , the redial data is displayed on the lcd display unit 33 and is stored in the dial buffer 29 , and , by the second input operation , the redial data stored in the dial buffer 29 can be used as a call destination number to request a call . preferably , the redial data is of the terminal number in the outgoing history to which a call was made most recently , but it is not limited to this . for example , it may be constituted such that a caller can select a terminal number from among the list in the outgoing history or incoming history . if the pressed key is the speaker key , it is checked whether there is dial data stored in the dial buffer 29 ( step s 17 ), and if dial data is stored ( step s 17 ; yes ), the dial data is set as a call destination number ( step s 18 ), and a call request message is sent ( step s 19 ). on the other hand , if dial data is not stored ( step s 17 ; no ), the procedure moves on to an offhook state . if the pressed key is the cancellation key , the dial buffer 29 is cleared ( step s 20 ), and the procedure moves on to a standby mode . according to the present embodiment , the transmission / redial key 32 is enabled to function as a combination of conventional transmission and redial keys , and hence the ip telephone set 10 is also enabled to have an increased number of functions . furthermore , redial transmission is possible only by operating the transmission / redial key 32 once ( in the offhook state ) or twice ( in the onhook state ) even if data is not stored in the dial buffer 29 . this provides a high level of convenience . the present invention has been explained , taking the ip telephone set 10 as an illustrative example hereinabove , however , the call request control device of the present invention can be applied to a digital telephone set such as an isid telephone set that sends dial data in batch mode . other variations , modifications and alternative embodiments of the invention will readily suggest themselves to those of ordinary skill in the art based on the disclosure herein , and therefore are to be regarded as being within the spirit and scope of the invention .