1. Create a translation rule to put an extra digit in front of the called number, say 1.
2. Add the translate-outgoing called to the voip dial-peer to the remote side
3. On the remote side creat a dial-peer pot with destination patterm 1....
4.1 will be striped and the real dialed digit would be forwarded.
On the reverse side use the same technique but prepend a different number.
This should help with you issue , but the real solution is to tell your PBX guys to
change the numbering plan.