This sounds like a dialplan issue where there's a timeout waiting for more digits. If the user dials a # after the number does the call complete immediately?
North American numbering? Are they dialing 7 digits, 10 diguts, or 1 + 10 digits?
I did compare the dial plan to a working site and didn't see any differences.
Hitting # they report it is slightly faster. North American numbering and dialing 10 digits.