SIP - > Interop -> Behavior on T.38 -> Invite Not Accepted
406: RE-INVITE for Clear Channel Only
488: Re-INVITE For Clear Channel Only.
606: Re-INVITE For Clear Channel Only.

SIP -> Interop -> Behavior on Machine Detection
Behavior on Machine Detection: Re-INVITE on Fax T38 Only.

Guardamos

Media -> Codecs 
El códec T38 debe estar con el valor Enable en la columna Data.

Guardamos y reinciamos la Mediatrix

Volvemos a Media -> Codecs y pulsamos el botón Edit correspondiente a T38
Enable: Enable.
Priority: 10. (*)
Redundancy Level: 3.
Detection Threshold: Default.
Frame Redundancy Level: 0.
No Signal: Disable.
No Signal Timeout: 1. (*)

Grabamos

Media -> Misc -> Jitter Buffer
Level: Fax / Modem.

Media -> Misc -> Machine Detection
CNG Tone Detection: Enable.
CED Tone Detection: Enable.
V.21 Modulation Detection: Enable.
Behavior on CED Tone Detection: Fax Mode.

Grabamos y reinciamos la Mediatrix

Esto es lo que tiene que tener configurado en la Mediatrix para que funcione los faxes por T.38.