From what I understand you want to redirect an incoming call to a specific number directly to another phone number. If this is what you want it is quit easy to do with asterisk.
I have an isdn2 connection with 5 numbers and an nt1 box with two connections.
Every number has its own extension so number matching with the numbr that has been called determines the routing of the call.
You can route the number of your choice to a context that will connect to another (internal or external) phonenumber. It should look something like below (in extensions.conf or an inlcuded file).
exten => ,1,Goto(redirect_incoming_call,s,1) ;
exten => s,1,Dial(Zap/g1/,20,rt)
You can call any number but the example uses one of the Zap channels in group 1. You also can add some additional lines in case the line is busy or unavailable. You also can add a second line that will make a call in case the first line isn’t available. In fact you can do whatever you think is usefull.
If this what you are looking for but you are missing the clue somewhere don’t hesitate to ask.
With friendly regards,
Erik de Wild