Here is one way. Excerpt:
“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.
The service, called TrapCall, is offered by New Jersey’s TelTech systems, the company behind the controversial SpoofCard Caller ID spoofing service. The new service is likely to be even more controversial — and popular.
“What’s really interesting is that they’ve totally taken the privacy out of Caller ID,” says former hacker Kevin Mitnick, who alpha-tested the service.
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. ”