This issue occurs when a low bandwidth codec such as g729 has been configured with the default fax rate voice setting.
To solve the problem of longer completion times, issue the fax rate command. This command configures fax transmissions to use a greater bandwidth than the codec compression.
To allow fax calls to negotiate to a maximum of 14400 bps, regardless of the voice codec configured, issue the fax rate 14400 command.
To provide deterministic bandwidth usage per call, issue the fax rate command within Voice-operated eXchange (VoX) networks. The fax rate voice setting is the default because it ensures that both voice and fax calls use the same amount of bandwidth within the VoX network.
Note: The fax rate voice setting is the default when changing the fax rate to something greater than the codec bandwidth.
Some fax machines may operate more stably at different rates than the default. In this case, issue the fax rate command to test the operation at different speeds.