This may happen when connected to certain ISDN networks. When Cisco CallManager is the user side of the PRI, it may send a Progress Indicator IE in the ALERTING message sent, which is invalid in some PRI protocols.
In the situation where Cisco CallManager immediately connects the call to Cisco Unity, the Alerting Progress Indicator may come after the CONNECT message is sent. This causes a protocol violation, and Cisco CallManager disconnects the call.
This issue is tracked by Cisco bug IDCSCdy26520.It is fixed in Cisco CallManager 4.0.
To resolve this issue, set the Disable Alerting Progress Indicator service parameter to True in the Cisco CallManager. This can be done by logging into the Cisco CallManager Admin page and performing these steps:
Go to Service > Service Parameters. Select CallManager.
Scroll down to Disable Alerting Progress Indicator. Set this parameter to True. ClickUpdate.