[dannzeman]

This seems like the answer to so many people looking for an app to do this. I would have thought this thread would have a lot more traffic. Here's a short explanation from Gizmodo:

from Gizmodo by Sean Fallon
A new service dubbed TrapCall allows users to unmask pesky blocked callers—revealing not only their number, but their name and address in some cases.
It's actually a fairly simple procedure. TrapCall reroutes unanswered blocked calls through a 1-800 number then back to your voicemail. Since anonymity is denied to 1-800 numbers, TrapCall simply picks up the number as the data moves through their servers. According to a Wired test, the rerouting process only took about 6 seconds on AT&T—and during that time all the caller hears is standard ringing.

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.

For a $10 upgrade to "Mouse Trap" service, users can receive transcripts of voicemail messages and can, in some cases, send text messages with the phone number of the caller. A $25 "Bear Trap" upgrade allows you to record phone calls and receive text messages with the billing name and street address of your caller. Amusingly enough, the only way to truly protect your phone number from TrapCall is by signing up for their SpoofCard service—it allows you to display a number of your choosing on a Caller ID.
Obviously, this will generate serious privacy concerns. On one hand, it prevents creepy or threatening calls and on the other, it could expose victims trying to keep their whereabouts a secret. So, which side of this debate are you on? [TrapCall via Wired via Boing Boing Gadgets]