text messages forwarded from my google voice number aren't recognized as being from my iPhone contacts

I set up my google voice number to forward to my AT&T number, but when I get texts from some people who are in my contact list, the text appears to come from phone numbers I don't recognize. The text message itself will be prefaced by the phone number the text is coming from, but even though that number is in my contact list, the message does not offer me the name of the person. I have an iPhone 4 and I'm running iOS 6.1. Any help is appreciated.