Fonality's faxing solution requires customers purchase and install 1 Sangoma PRI card and 1 Sangoma analog card. You cannot use a fax machine with
PBXtra
with only an analog card and analog lines. Your fax machine requires it's own extension. Fax over VoIP is NOT supported.
The major hurdle with Fax + an IP PBX until now has been proper timing for the fax machine. Without precise timing, the actual facsimile will become corrupt during transmission and the resulting page that prints will become unreadable.
Sangoma has provided a solution which allows an analog card to pull timing from a T1 card. This feature allows faxing over a T1 line through an FXS port to a Fax Machine.
There are 2 timing pins on the top back corner of all Sangoma Cards (T1, Analog, BRI). These pins transfer timing information from a T1 card, where timing is provided by the Telco, to an Analog and/or BRI card. The pins must be connected 1-1 and 2-2; therefore, Fonality used a straight 2 pin cable often used as a CDROM Audio cable. A T1 card can can actually provide timing for multiple cards; however, Fonality currently only supports a single analog card used for Fax machines.
In the image below, the fax sync cable connects to the back of the analog card (towards the back) and the back of the PRI card:

If "RM_NETWORK_SYNC" is set to yes, there must be a timing line present. The wanpipe drivers will not load if the cable is missing.
If the carrier does not provide precise timing on the PRI card, the analog card will not properly send or receive facsimiles. Please double-check the T1 configuration before escalating any issues.
For full details please refer to Sangoma's Wiki Page:
Sangoma Wiki Article