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Fax transmissions are slow, incomplete, and corrupted when a fax is sent at either the high speed baud rate of 144000 bps or at a low speed, such as 9600 bps

Core Issue

Faxes fail in this scenario:


All the T1 Primary Rate Interface (PRI) lines on the originating and terminating Cisco CallManagers run clean. There are no clock slips, and Path Coding Violation (PCV) and Line Code Violation (LCV) are counted. All T1 lines derive the clock from line (clock source line).

Faxes always work from VG224 to Cisco CallManager, but fail at high speeds, such as 144000 bps, from CMM-VG01 to CMM-VG02. Partial fax pages or corrupted lines in the middle of the page occur, even when the fax speed is set down to 9600 bps and all pages are received. Modem passthrough switchover behaves properly and there is no packet loss in the IP network.


In order to resolve this issue, configure clock source line primary on both the originating and terminating Cisco CallManagers under the appropriate controller.

Refer to
Clocking Configurations On Voice-Capable IOS-Based Platforms for further background information on clocking configurations.

Open a case with Cisco Technical Support with the
TAC Service Request Tool if the issue is not resolved.