outbound route from inbound route

Hello,

I'll start with a scenario:
I have calls come in from one trunk, that I want to go out to a PSTN number sometimes but I want it to go out over a specific trunk when they have come in from this trunk. Does that make sense?

So call comes in trunk A, PBX does some stuff, maybe plays a greeting, holds them in a queue and then calls a PSTN number (or classical extension). However, I want it to use trunk A for the outbound, not any of the others. I have quite a few trunks and quite a few other classical extensions that I want to call, but this one classical number must be called by this trunk.

The only way I can think of doing this is prefixing the PSTN number with a junk string, then using the outbound route to match this, then stripping it off before passing it to the trunk.

Any other ideas?

Thanks