VoIP Users Spoof Caller ID
Kevin Poulson, writing for Security Focus points out a dirty little secret about VoIP -- that users can fake their "calling party number" or Caller ID. In addition, consumers who "block" their caller ID can find that hackers are able to pick this information up from the network.
The underlying issue appears to be the insecure implementation of CPN in the traditional telecom companies' networks. The system "...relies on telephone equipment at both ends of the call being trusted: the phone switch providing you with dial tone promises not to lie about your number to other switches, and the switch on the receiving end promises not to reveal your number if you've asked that it be blocked."
This will likely become another argument for regulation of the emerging VoIP industry, though this will only stop legitimate companies from these practices. A better solution would be to revise CPN standards to be more secure.