Before addressing issues with analog Foreign Exchange Office (FXO) or Foreign Exchange Station (FXS) circuits, ensure that your configuration is correct and that your hardware supports these circuits.
For more information, refer to these Cisco TAC Case Collection cases:
Some of the common issues observed with the analog FXO and FXS circuits are documented in these cases:
In CallManager, the analog FXO and FXS card is not recognized by the gateway
For more information on no dial tone issues, refer to Troubleshooting No Dial Tone Issues.