Trapcall which in beta and the basic service which is free is offering to unmask those masked calls. The service currently runs on phones serviced by AT&T and T-Mobile and plans to carry the number unscreening to other carriers as well.
The simple trick of forwarding your unanswered blocked callerID call to a 1-800 number and viola, you have the caller number.
Another part offered is, opportunity to record all your calls. There is another service send the transcripts of voice mails delivered to you via SMS. This will be better way to get your voice mail even when you are in a meeting, class, movie. (The transcription is done by hand and signing up for the sevice, you are allowing TrapCall to listen to your voicemail.
After finding the blocked number, you will also be able to block the number if ever need to. Once that is done the caller will recieve "number disconnected" message.
According to the site;
"Trapcall works on any standard cellular phone that is registered to a carrier. All you have to do is sign up for an account. Start experiencing Trapcall today, for FREE by Signing Up.
Even though Trapcall works with any standard cellular phone, you must contact your carrier to view charges that may occur, or to get Call Forwarding Busy activated for Trapcall's features to work correctly."
So if you are in need of finding out who is calling you anonymously, this is one way. But if you are the caller, according to Gizmodo, you may want to sign up for spoofcard, which allowes you to spoof a number of your choosing at the time of calling.