T.38 seems to be tricky. At least Sipgate says:
Does sipgate work for fax numbers?
We have had mixed feedback from users on this one. We currently do not support the T.38 codec which is designed for fax transmittal.
Fax machines are finicky and even an imperceptable delay (using another codec) can result in the fax machine stopping transmission.
Some people report success and others not. We plan to add this codec and offer fax service, but currently it is not integrated.