I dont know the exact format for US-Fax numbers, but in germany it is common to write it in more than one "fashion".
e.g. 040-1234567, 040/1234567, or the international version which applies for you too (there the "0" from 040 is removed) ?: +49401234567.
Maybe this is similar for US? First thing would be to see what are common, allowed forms of the fax-number then build the regex i would suggest.
Update: People might even write it like these: 040/1234-67 or 0 40/ 12 34 67 ... So i think it could be (like others mentioned) getting complex to match all these, maybe some "simple form" could only check for allowed signs (-, /, whitespace , +, digits ...) so that no letters are used at least?