We are succesfully receiving faxes with V4 integrated fax server.
We can only send from an extension if we use G711 voice codec and fax passthroug. We have not yet succeded in using T.38.
PSTN Gateway is PATTON 4554.
In the Gateway, we have created an adittional SIP GATEWAY, all the external calls to the FAX number are routed to this SIP GATEWAY.
Patton Smartnodes can do this type of thing.
This way we can tell 3CX that all the calls coming from this particular SIP GATEWAY are fax calls.
We didn't try the config file generated by 3CX, but we looked at it, and I suspect it was not correct (T38 was not correctlyactivated).