When configuring the Fax Mode, you should pay attention to the selection of Voice Codecs. The default SIP Voice Codecs or H.323 Voice Codecs configuration can be overridden by the Outgoing Voip Call Routing and Incoming Voip Call Routing rules.
When selecting voice codecs, you should be guided by the following rules:
- T.38 fax is fully compatible with using of any audio codec. At least one audio codec must be selected. At least one of the selected codecs must be supported by your SIP or H.323 network.
- G711 fax can only operate when using G.711-ulaw or G.711-Alaw codecs. Thus, at least one of these two codecs must be selected and supported by your SIP or H.323 network.
- To prevent sending or receiving faxes in the audio mode, you can disable G.711-ulaw and G.711-Alaw codecs. This is possible if your SIP or H.323 provider supports one of the other voice codecs.
- To provide compatibility with audio fax when using codecs other than G.711-ulaw and G.711-Alaw, it is strongly recommended to use the highest preference for G.711-ulaw and G.711-Alaw. When Fax Voip T.38 Modem switches to G711 fax mode, it makes re-invite to G.711-ulaw or G.711-Alaw, if necessary. Re-invite to G.711 is fully supported in Fax Voip T.38 Modem in the case of incoming calls. However, in the case of outgoing calls, this feature is not finalized, so you should not rely on it. In the case of SIP, you can also provide the highest preference for G.711-ulaw and G.711-Alaw codecs for incoming calls. To do this, you should use the highest preference for G.711-ulaw and G.711-Alaw and additionally check Apply codec preference order to incoming calls option (Note, that this option is not recommended).
- Dummy G.729* codec enables to report G.729 Audio Codec support for Fax T.38 Gateways that do not support G.711 and other supported audio formats; use this codec only for resolving compatibility issue. Typically, if dummy G.729* codec is used, T38 Fax with Re-invite mode for outgoing calls should be used too to ensure CNG tone generation.
