Do you block your phone number from appearing on Caller ID? If so, don't count on it. At The Last HOPE hacker conference, Kevin Mitnick, arguably the most famous hacker of all, demonstrated how call blocking can be hacked, and the hidden phone number exposed.
The hack starts with a VoIP telephone number. Mitnick uses Flowroute as his provider, but he told me afterwards that the same thing can also be accomplished with a few other VoIP providers.
He starts by forwarding calls to an Asterisk server that he maintains.