Interesting. Could you describe the procedure? I started at porting from Google to another (Callcentric) and didn't know how to provide the billing documentation they asked for. What do you ask people to provide?
At this point, you fill out a form asking for the billing information and then we follow up with you to get authorization to transfer the number (usually just a signature). It usually takes about 10 business days to complete the port.
(I work at Twilio too)