Not everyone uses GV for the same reason(s). The point, as it is with many topics, is subjective. For me it would defeat the purpose but that doesn't mean that it would for everyone else. In the OP's case it doesn't. OP just wants to keep using the existing number.
http://support.google.com/voice/bin/answer.py?hl=en&answer=1316844
Try starting here. Don't overlook Google -- which is how I found this.I'm leaving in Akron, OH (area code 330), even my billing address of AT&T is for here. I have a Google voice number of San Diego County, California (area code 858). I am in a 2-year contract and want to port my Google voice number to AT&T within the current contract. How can I do that? Please help. Thank you.
http://support.google.com/voice/bin/answer.py?hl=en&answer=1316844