Cannot fax outbound but can receive

That error code is actually a communication error caused by no response from the called station or a misdialed number. Make sure the number is correct and redial it.

Receiving fax never picked.

It means No Good. 5 possibilities:

Receiving machine unplugged
Not a fax number
Wrong fax number
Busy signal
Digital Phone Line

receiving machine is always BUSY

press the monitor key...you should hear a dial-tone...if not try below.

Check the phone line with a regular phone plugged into the wall jack.

error code 634 means that your machine tried a phone number and reached its redial limit. does this happen to every call or just one number.
if its all the time it could be you phone line is not connected or you need to do an all job clear

The receiving fax machine is turned off or not functioning. Try calling your cell phone.

Do you have a DSL phone line?

error code 634 means that you have reached your limit of redials

error 634 is an indication that the line was either busy, or there was no answer on the receiving end.

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.