Since it sounds like you have a phone line and a fax line, I'd do a one time switch: connect the fax machine up to the regular phone line; then have your client try to fax you on that line. If you receive it just fine, then it's probably something with your phone company.
I have an SCX-5739fw multifunction machine set up. I can send faxes to one of my clients, but I cannot receive any from her. Her fax number is the only one this happens with. She can send faxes to anyone except me and I can receive faxes from anyone except her. When she tries to send a fax to me, she just receives the "this line has been disconnected" message. I have gone many times between my cable provider and Samsung, but have not come close to solving this problem. I don't know what I haven't tried. Can it be as simple as trying a different number? This did not happen until I moved my office to my home. I was able to keep my phone number, but I had to get a new fax number.