Not having a dial tone can be caused by a hardware failure, an incorrect configuration, or a software issue.
If the Cisco IOS gateway is correctly configured, and there is no dial tone from the router when the phone or PBX goes off-hook, perform these steps:
Check to see if the voice ports are in administratively shutdown mode. In this mode, no dial tone is played. To check the dial tone, issue the show voice port [slot-number] command.
To enable the ports, issue the no shutdown command under voice port mode in the configuration.
Make sure the direct-inward-dial command is not configured under the dial-peer voice pots command. This is not supported on analog Foreign Exchange Office (FXO) and Foreign Exchange Station (FXS) cards. Direct-Inward-Dial (DID) is only supported on Voice Interface Card (VIC)-DID cards. If DID is configured on the normal FXO and FXS cards, it can disable the dial tone and usually returns a fast busy signal. Remove this from the configuration and try again.
Check to see if you have the Private Line, Automatic Ringdown (PLAR) or trunk configured under the voice port. This disables the local dial tone as well. To test for the local dial tone, temporarily remove the connection plar or the connection trunk commands. Reset the voice ports by issuing the shutdown and no shutdown commands and check for the dial tone again.
Check to see if the dial tone is being played but not heard by the remote phone. Issue the debug voip ccapi inout Cisco IOS Software command, go off-hook, and look for a ccGenerateTone function in the output. If ccGenerateTone occurs in the output, the router is generating a tone. This is a symptom indicating that the transmit and receive pin-outs may be wired incorrectly.
If you do not see any debugs, check to see that they are enabled to play on the console. Issue the terminal monitor command on the command line and issue the show debugging Cisco IOS Software command. Make sure the debugs are enabled. Issue the debug vpm all command to monitor for voice port Telephony signaling. If you do not receive any debugs, this could be caused by mis-wiring on the transmit and receive pins. Verify the RJ11 tip and ring wiring.