Kevin Poulsen on TrapCall:
A new service set for launch Tuesday allows cellphone users to unmask the Caller ID on blocked incoming calls, obtaining the phone number, and in some cases the name and address, of the no-longer-anonymous caller. […] TrapCall’s basic unmasking service is free, and includes the option of blacklisting unwanted callers by phone number. It also allows you to listen to your voicemail over the web. It’s currently available to AT&T and T-Mobile subscribers, with support for the other major carriers due within weeks, says TelTech president Meir Cohen.
Sounds great — Exactly what I’ve been looking for.