The default meaning of the 503 message is understood to be service unavailable in the world of ISDN. In case you receive the 503 disconnect cause code as no circuit available into the gateway, tell the gateway to view the 503 cause code as ISDN cause 34 no circuit available. The calls are not routed due to the mismatch in disconnected cause codes.
In order to resolve the issue, make these configuration changes on the voice gateway:
- configure terminal
- set pstn-cause value sip-status value
Refer to the Error and System Messages section of Troubleshoot and Alerts guide for Cisco IOS Software Releases 12.4 T.