The Phone Number Roach Motel
Tom Keating has a rather long-winded description of his issues with Number Portability between VoIP, PSTN, and Mobile Phone providers. The bottom line is that porting your number into a VoIP provider is a bit like the old Roach Motel ads: you can check in, but you can't check out (port it out of a VoIP provider).
The big problem: the laws regarding local number portability aren't equitable. Landline and mobile carriers are subject to very specific laws, the VoIP carriers are not. VoIP carriers are not required to port numbers, though many of them do as a courtesy for their customers. They'll take any number in. Ask them to give up a number they've assigned you, and you can forget it.
Of course, the whole concept of phone numbers seems so outdated anyway, but like that concept of "minutes," we're stuck with them for the forseeable future.