When this device is "on hook", the speech circuit is disabled & the ringing circuit is connected
... 
an inhibitor device responsive to the incoming call indication and the off-hook .... 
a valid dial tone input connection for receiving a valid dial tone... [DOC] [TLE] AN1603 Providing a POTS Phone in an ISDN or Similar Environment [PAR] of the Ring Generation portion of the circuits for the purpose of lower cost ... state 
(phone onhook, not ringing), the relay connects the phone line to the ... loop 
current, and the speech paths between the phone and the CODEC.