The numbers are a part of a trunking group - need to do it on the PBX or else we won't get a 1:1 forward.
I'm not completely sure what you mean by 1:1 forward -- but if you use the Toshiba to do the forwarding each incoming call will tie up two lines. Unless your customer has a PRI or quite a few COs, outside callers may start getting busy signals because other incoming/forwarded calls have already seized all your lines. You will most likely have issues with low volume on the forwarded calls also.
You won't have to worry about these potential problems if you let the telco do the forwarding, which is what I and most others always do when in your situation. If you're still set on doing it from the old system, the best I could offer you is this link where you may be able to obtain a manual for it.
www.wedophones.com