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.
The digits can be any set before the 5. I dont know what ARS is. When I add new numbers to the phone system I dont mess with the trunk group (maybe I should be adding them, this Intertel is all new to me) It has happened with 2 separate numbers and still going on this morning. The only thing the two numbers have in common are the 5 as the seventh digit. Im not sure what you mean by "what kind of lines" do we have. I had the user attempt to dial 92000 + <phone number> when he dialed the digit following the 5 it says "unable to route call"
My number was in the system before some of the new numbers and I am also having the same issue when I dial from my desk phone as well.