Regarding billing, Google Voice is a call-forwarding service, so your AT&T bill will show charges for calls you make with your phone even if the calls are dialed through Google Voice. Google Voice is connecting you to a desired party through its forwarding services, but you are still using your AT&T service to actually complete the call and you are charged accordingly.

AT&T Phone Plan Charges

If you place a domestic call through Google Voice, your phone's call log will display the direct access number Google Voice used to connect you with your desired party and the call will use the standard minutes as dictated by your AT&T cell phone plan. If you dial an international number through Google Voice, the international call will still use the standard minutes from your cell phone and will be additionally charged against your Google Voice credit at the relevant international rate.