Is this true no matter what the digits before the 5 are? Are you using ARS or selecting a trunk group? Since that 5 is the seventh digit dialed it sure looks like the number is being treated as a 7 digit local number. What kind of lines do you have? Instead of dialing 8 or OUTGOING try dialing 92000 (ARS) and the number and compare what happens if you dial 92001 (or whatever your trunk group is) followed by the number, see if the behavior is the same or different.
None of this is ti fix it, just to help determine what's wrong.