The Gateway can operate by either leaving the fax audio as regular in-band, or by converting it to T.38. If it's T.38, the other side (what it's talking to in SIP-land) will need to be able to handle it and do whatever it is you're expecting it to do. If it's in-band, then it's just regular audio that happens to be a Fax. The gateway isn't doing any magic, it's just a dumb transport. There's no fax to activate on it.
Digium, Inc. | Senior Product Manager