the answer is, it depends. Who is sending you the call? What protocol(s) are they using? What is your basic application?
Your question is not really basic since the forwarding mechanism will depend on the caller’s protocol of choice. Lets say its H.323 and you are using GKs, you can simply use a GK to match a specific E.164 address and respond with the corresponding IP address through LRQ.
If it is SIP, then you can create a proxy that will forward the invite to the proper IP. If MGCP, you would have to rely on the call agent’s forwarding mechanism.