DID Forwarding Setup?

I'm looking to setup a system to resell DIDs, but I'm unsure of the best way to proceed. I've tried to implement OpenSER thinking that a softswitch may be the best solution. I'm in the process of trying out A2Billing as well.

What I'm thinking that I'd like to have is a DID translation table that will take an incoming call on a DID look up what PSTN number translates to and forward that call (without proxying the media for greater volume... or would proxying be necessary for this setup?), billing for the whole call once some time threshold has been reached. Any suggestions as to how to approach this? I thought to build this system using OpenSER for its large volume capability.