Core Issue
This problem affects the Cisco 3745 gateway router, which has several Foreign Exchange Station (FXS) ports installed. When the FXS port is used to make calls toward the Cisco CallManager, the calls fail, and the caller hears a busy tone. However, calls made from the IP phones to the FXS port work as expected.
Resolution
Normal Cisco IOS Software operation is to hunt through configured dial-peers in a top-down fashion. So when the FXS port goes off hook, Cisco IOS hands over voice port call control to the default H.323 gateway or session operation. Since the router is not able to make a match on any digits dialed by the handset, it plays the re-order tone and calls fail.
An inward call from the IP network matches on the dial-peer that has the application mgcpapp command configured, and can be sent out of the voice port.
To resolve this problem, perform this procedure:
- Make sure the application mgcpapp command is configured on the applicable port.
- Re-order the Plain Old Telephone Service (POTS) dial-peers to list the Cisco CallManager-generated dial-peers first in the dial-peer list. Ensure that inward and outward call control is handled by the correct dial-peer under Media Gateway Control Protocol (MGCP) control.
- After the MGCP is reset on the gateway (issue the no mgcp and mgcp commands), make a few test calls.
Ideally, calls are possible in both directions through the FXS port now.
For more Information about how to configure the MGCP gateway, refer to Configuring MGCP Gateway Support for Cisco Unified CallManager and How to Configure MGCP with Digital PRI and Cisco CallManager.
