Faxes failing the negotiation process. When ringing through, the fax starts to signal, however after a couple of seconds, the signalling is supressed, followed by an error on the Fax Display or results sheet. The Disconnect cause is 65 – Bearer Capability Not Implemented.

Setup Traces in CUCM and display the output in the TranslatorX. TranslatorX displays the key information of a call leg within a couple of mouse clicks. Below we can see the an Internal Fax is attempting to fax out to the PSTN. The fax is connected to an ATA Device.

Due to the nature of faxes, the G729 codec is not compatible with fax signal. The G729 compresses the rtp and hence corrupts the fax transmission. Below we see the negotiated codec is G729. So we’ll have to check how this codec was negotiated and not the G711a/ulaw.

Check the codec negotiated.

1. Check the Device Pool of the ATA and Voice Gateway. Collect the Regions, the confirm the Regions between the ATA and the Voice Gateway can indeed negotiate G711a/ulaw.

2. If all looks good, log into the Voice Gateway and check the codecs configured for the dial peers. Voice dial-peers on IOS Voice Gateway default to G729 codec. You must configure a voice-class with a preference to use G711alaw first. This ensures G711alaw is negotiated between the ATA (Fax) and the Voice Gateway. In the below example, only the codecs G711alaw and G711ulaw are permitted on the dial-peers 10 and 11.

** With the latest CUCM Deployment models, there is also the option to create/modify Codec Preference list under the Regions Menu. This allows us to the preference codecs between endpoints registered to CUCM and/or the Voice Gateways leading to the PSTN Network.