The Gamma SIP Trunk Service will support Fax and Modem transmission subject to the following constraints
- FAX and Modem transport in band using G.711 a-law codec is supported.
- Renegotiation to T.38 is supported. (subject to interoperability testing)
The use of G729 for in-band faxes is not supported as its compressed nature may cause tones and messages to be lost.
If the fax option for an endpoint is set to T.38 enabled then:
- The Gamma network will attempt to re-negotiate [re-INVITE] to T.38 for fax calls for ingress (Customer to Gamma) calls on detection of a fax tone.
- The Gamma network will accept a re-negotiation [re-INVITE] to T.38 for fax calls for egress (Gamma to Customer) calls.
- The re-negotiation must be done using the re-INVITE mechanism after answer.
- Due to different vendor implementations Gamma cannot guarantee T.38 interoperability with all vendors and will not accept responsibility if T.38 interoperability cannot be achieved with a specific vendors implementation.
If the fax option for an endpoint is set to T.38 disabled then all fax calls will be handled as G.711 pass through providing the customer has a G.711 codec available in his media profile.
Ensure your endpoint’s audio profile is set up for T.38 relay. Upon the detection of fax tones, a T.38 re-INVITE will be transmitted. If a 200 OK T.38 is not issued in response to this re-INVITE, the T.38 handshake will not take place, and the fax transmission will fail. Gamma supports T.38 v0 only.
Please note the SIP Trunk Call Manager platform doesn’t support fax, the alternative is to use fax divert.
