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PRI calls are disconnected intermittently, and calls to 1800 numbers that come over an MCI trunk hear a fast busy
This can happen when connected to certain Integrated Services Digital Networks (ISDN). When Cisco CallManager is the user side of the Primary Rate Interface (PRI), it can send a Progress Indicator (PI) information element (IE) in the alerting message, which is invalid in some PRI protocols. In the case where Cisco CallManager immediately connects the call after this, such as when forwarding to voicemail or answering with an auto attendant, the ISDN network complaint about the Progress Indicator can come after the connect message is already sent, which causes a protocol violation, and Cisco CallManager disconnects the call. This only happens with a Media Gateway Control Protocol (MGCP) controlled PRI gateway.
In order to fix the issue, change the DisableAlertingPI service parameter to True in all servers in the cluster, because when Cisco CallManager is configured as user side with the DMS-100 protocol, it must not send a PI in the alerting message.
See this call scenario:
1.NETWORK USER NETWORK 1.Setup---------> 2. (CFA) Setup-----> 3. CallProceeding<-------- 4.CallProceed<--- 5. Alerting<-------------- (with PI) 6.Alerting<------ (must without PI)