Incomplete info code 634 how to fix

That codes says:

"Redial count over with no response
or busy tone was not detected.
Note:
U.S.A. and Canadian models
will redial only once if a busy
tone is not detected."

The number you're dialing was first busy. When it went to redial, it detected no phone signal on the other end. If it happens to only 1 or 2 customers, then it's something to do with their lines and you can't fix that.
If it happens often, then get your phone lines checked out by your phone company, make sure your fax is not sharing the line with another other device and make sure it is not a digital line.
Other than that, you're looking at having to replace the main board in the fax. Which basically means it's time for a new fax machine.