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Dropped calls are experienced intermittently when calling a mobile phone through a PRI connected voice gateway

Core Issue

When a user places a call to a cellular phone and receives the message "please wait while we locate the subscriber," the cellular phone drops the call intermittently.  Non-cellular phone calls are fine.  The public switched telephone network (PSTN) connection is via a Cisco Catalyst 4000 Access Gateway Module (AGM) blade and a PRI.

The problem is related to the PRI not receiving an updated call_proceeding message within the timeout period.


Resolve this issue by adjusting the timeout on the gateway for the ISDN t310 timer.

Set the timer to a high value, such as 60000. This value makes the timeout 60 seconds which should be long enough for any network. The timer is found under the serial interface configuration on a Cisco AS5300, 2600, 3600, 3700, and other IOS  gateways such as the Catalyst 4000 AGM.

Telephony Signaling Troubleshooting TechNotes

TDM Signaling Type


Voice Gateways

IOS gateways