6505 Fax issue

I have a 6505 that will not send or recieve a fax. When you send a fax to the device it answers the call then hangs up on you. When you try to fax scans the docuent and dials bit it never seems to really go off hook. The device is connected to a grandstream ata. I have tried turning ecm off, setting baud to 9600, swapped phone cords. Connecting a phone to the line allows me to place outbound calls and recieve calls, and connecting a different manufacurer multi function fax works also. Any suggestions would be great!