I have a WC7556 that will not fax. I have reset all software multiple times, ensured that the fax settings are set to "Send and Receive," and have ensure that my phone cable and phone jack are working .....help?!?
First thing with fax, call it on the phone, if it doesn't answer the phone cable might be bad, if the Phone company has a message listing the account has issues, if it picks up and screeches like tortured pig the machine is at least acknowledging the phone line coming in.
Then do the same thing at the device by listening to the outgoing fax. To do this log in as the admin and go to Machine Status > Tools > Service Settings > Embedded Fax Settings > Transmission Defaults > Audio Line Monitor and turn it up full, then apply and log out and send a fax, you will be able to hear it pick up the line, get the dial tone, dial the number, hear the other end pick up, the handshake, the transfer etc etc.
That should give you a very detailed reason for the failure.
Of course if nothing happens at all on the send or recieve it would likely mean the fax board itself has died.