Fax and DTMF support
The 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 network will attempt to re-negotiate [re-INVITE] to T.38 for fax calls for ingress (Inbound) calls on detection of a fax tone.
- The network will accept a re-negotiation [re-INVITE] to T.38 for fax calls for egress (Outbound) 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.
- If calls are made to another CP that does not support the method of transport for the tones, there will be no conversation between the two different methods.
The following methods will be supported to transport DTMF tones:
- The core network will support the generation of ‘In-band’ or ‘RFC2833’ DTMF transport based on end to end negotiation.
- RFC2833 is the preferred method for the transport of DTMF tones. Support of RFC 2833 is dependent on successful codec negotiation and requires the payload type 101 to be assigned. RFC2833 will be used with both G.711.and G.729 codecs.
- In band over G.711 codec only
- If a G729 codec is being used then DTMF tones should not be sent in-band, we will not guarantee the delivery of in-band DTMF over a G729 codec.