If you are about to go over on your minutes you are notified when you receive a call what network the caller is on (i.e. AT&T, Verizon, etc.) Since in-network calls don't count against your minutes you'll be able to identify and answer those and not answer the other calls.
Exit strategy - Sell to Google (like everyone else) and be added to the functionality of Google Voice.
or take it to another level with caller ID like residential lines... Regardless if the incoming caller is in your contact list... App identifies number and provides you name as listed in phone directory. Now *thats* a service I'd pay for :)