The T38 to G711 Fallback Procedure

If the SIP External Gateway configuration parameter is “T38 to G711 Fallback” and if the IP Couplers in front of FAX are INTIP3/MG3 couplers and if the initial call is established with G711, OXE will try to establish the FAX in T38 mode.

If the remote SIP Party is not able to support FAX in T38 mode, it will send Error message. This will result in OXE to switch the FAX to G711 Mode.

Outgoing call
If OXE receives a RE-INVITE with T38 parameters, the call will proceed in T38. If OXE receives FAX call in G711, it will directly detect and handle it.

Incoming call 
Step1: When OXE detects a T38 FAX call, it sends Re-INVITE with T38 parameters as usual.
Step2: If the SIP Carrier accepts it and 200 OK is received with T38 parameters, then call proceeds in T38 mode.

Else if the SIP Carrier does not accept it and sends an Error response, the following cases are envisaged: -

• Case 1: If the negotiated codec is G711 and the IP couplers are INTIP3/MG3 couplers, then OXE will switch to G711 mode. 

• Case 2: If the coupler in front of FAX is other than INTIP3/MG3 coupler, or if the negotiated codec is G729/G723, the call is disconnected. 

Remark: If OXE is in transit position, the Error response will be relayed transparently