Hi Jason,
There are some settings under the fax-relay configuration - here you would try to disable fax-relay and set the port to voice calls only. Alot of times the issue is between the various types of error correction mechanisms (ECM) that different fax manufacturers implement. So when you set it to "voice-call" or "voice pass-through" the Cisco gear wont try to negotiate ECM.
Let me know if you have trouble finding these commands under the fax-relay config - I don't have one in front of me.
Cheers,
Erik.