When I first got Vonage service a couple years ago, Murd0c just called up their customer service line and said "What's Brad's number?" and they were more than happy to give everything up on me.
I also managed to get someone's full info on their J-Fax account once. Not exactly VoIP, but it was still really easy. I've gotten information from a number that was with Comcast Cable too. I don't think any of it is that much harder than regular numbers, really. The hardest part is just figuring out which VoIP provider a number is with.
