The current restriction to 10 digits for the fax number field on the outgoing fax page is certainly computer friendly, since the number is in perfect form for dialing. Unfortunately, the restriction is not human friendly. Humans usually will copy the phone number from some other app, and would like to paste it into the web form. However, the phone number usually has extra characters, like spaces, hyphens, etc.

It would be handy if at least you could paste the number in and then manually remove the extra characters. Currently, that doesn't work since the number gets truncated due to the field length. However, even the field were longer, the human would ask, "Why am I removing the extra characters by hand? Isn't this the kind of work computers were invented for?"

My suggestion is to make the field really large and allow anything. Include a simple message like, "Any non-digit characters will be ignored." If you want bonus points, include some JavaScript to remove the extras when the fax number field loses focus.