When configuring the Fax Mode, you should pay attention to the selection of Audio Codecs.
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.
- Please note that depending on the T.38 fax settings some initial fax operations can be performed in the audio mode before switching to T.38. If you use codecs other than G.711-ulaw or G.711-Alaw, CNG and CED fax tones can be not transmitted and not detected. That is why we recommend to use G.711 codecs even if you work with T.38 faxes.
- To ensure CNG tone generation when sending T.38 fax and audio codec is not G.711-ulaw or G.711-Alaw, you can use Switch to T.38 after ... seconds T.38 fax option with small delay (0-2 sec).
- Audio fax (Fax over G.711 codec) can only operate when using codecs G.711-ulaw or G.711-Alaw. 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. The correct re-invite to G.711-ulaw or G.711-Alaw when starting the call using other audio codec is not guaranteed in the Fax Voip FSP application.