We use this tutorial for deployment of Cisco ATAs for faxing.  Be sure to update the ATA firmware to the latest available, and set to static IP. When using DHCP these devices tend to stop working from time to time and require a reboot.

• Select Voice in the Menu Bar, and then select either Line1 or Line 2 depending on which line you have used for your fax, then select Network. Set the Network Jitter Level to very high and the Jitter Buffer Adjustment to no.
• Next, in the Supplementary Service Subscription section, select the following settings; Call Waiting Serv to no and the Three Way Call Serv to no.
• Navigate to the Audio Configuration section and enter the following settings to support T.38 Fax Preferred Codec; G.711u (USA) or G.711a (rest of the world). Select Use pref. codec only as yes, Silence Supp Enable as no, Echo Canc Enable as no,and FAX Passthru Method as ReINVITE.
• Click Submit to save your settings
• Since you are are using the Cisco media gateway for PSTN termination; then Disable T.38 (fax relay) and Enable Fax using Modem Passthrough. For example:
  modem passthrough nse payload-type 110 codec g711ulaw. Next set fax rate to disable, and fax protocol pass-through to g711ulaw
  Note: If a T.38 call cannot be set-up, then the call automatically reverts to G.711 fallback.
• Verify that your fax machine BAUD rate is set to a speed between 7200 and 14400. We suggest 9600.

Try to fax as a test and make sure it completes.