The issue is addressed in Cisco bug ID CSCdy26520.
This issue is observed when connected to certain ISDN networks. When Cisco CallManager is the user side of the Primary Rate Interface (PRI), it sends a Progress Indicator IE in the alerting message sent, which is invalid in some PRI protocols.
In the case where Cisco CallManager immediately connects, the call after this, such as when it forwards to voicemail or answers with an auto attendant, the ISDN network complaint about the Progress Indicator comes after you already send the Connect message, which causes a protocol violation, and Cisco CallManager disconnects the call. This only happens with a Media Gateway Control Protocol (MGCP) controlled PRI gateway.
Disable Alerting Progress Indicator This parameter determines whether the alerting progress indicator to Inband Information is reported to digital PRI gateways. Valid values specify True (disable the alerting progress indicator) or False (send the alerting progress indicator).
This is a required field. Default: false.
In certain configurations, in order to receive ring back, you must set this field to False in order to force media cut-through.
Complete these steps in order to resolve this issue:
Change the service parameter Disable Alerting Progress Indicator to True.
Use the Cisco IOS Software Release 12.1.3*T*, where the issue does not exist.
In case this issue is not resolved, use the TAC Service Request Tool in order to open a case with the Cisco Technical Support.