Not a lot of info can be given because its a phone line.
My guess is you have digital/Voip/FOIP lines and the device doesn't support them. There may be a fault code to help out, look for that. I don't have a 3215 to check, but most devices are able to let you listen in, via a Fax Volume or by hooking up a wired phone to the machine itself.
I recently purchased and set up my Xerox WC3215 multifunction printer. When someone is trying to fax, the unit rings as set (2), says, "Receiving", but then beeps with the message [COMM. Error] . What does that mean, what should I do, what can reset the machine? It sensibly means "Communication Error", but what, exactly? thanks for feedback