Porting my phone number to Google Voice was a three day process, which I was pre-warned about. The mobile carriers in the U.S. have made the porting process between them fairly easy, and it occurs over a couple of hours. But they are in no hurry to help customers move their phone numbers to Google Voice, and so it took a few extra days. Also, I'm one of the first people to port their phone number to Google Voice, and there are always a few hiccups when you're a guinea pig.