Instead of changing channel/routing, why not just treat the DID as it comes in? ie build an ACD with NCFW to where you want the call to go, or CDP's etc, there are several other options that can be used to do this without cutting down on the number of trunks available to everyone else.
The routes you build in the PBX only deal with outgoing calls, not incoming. If you still want to segment your circuit, as allenmac says, you will need to get the carrier involved to make changes their end and remove the members from your existing trunk to stop outbound on those channels in the current config.