You could also setup some matching rules in ARS, use a digit modification plan to strip out all those leading digits, and then send the call out a looped trunk group (If you have a spare dual E1 card, looping the two ports, and creating a tie trunk). If you already have two PBXs, then just strip the digits and send it over to the other PBX via IP/XNET, and let it route it back (as long as neither PBX complains of tromboning)
Sounds a little crazy, but at least it'll just "work" no matter what the users dial, without having to tell them how to dial, or give them reorder.
There's GOT to be a more elegant way rather than looping a T1/E1 back though, or just flat denying/RADing...