Maybe nobody cares about this but me but I got to wondering. Since I have my Google Voice set to pass through to me the Caller ID of the caller, does AT&T Wireless think that the incoming call is really mobile-to-mobile?
Here's my Google Voice setting relative to incoming Caller ID.
Here's the entry in the Google Voice History for an incoming call from an AT&T mobile.
Then here's the entry in the AT&T call log showing a matching number, time, and duration and indicating it was "M2MCNG."
Just to close the loop, here is AT&T's definition of "M2MCNG."
What this means is that you can share your Google Voice number with everyone. If someone then calls your Google Voice number with an AT&T mobile phone and you answer it on your AT&T mobile phone that call will be counted as mobile-to-mobile, i.e. free.
If you're not an AT&T Wireless customer, your mileage may vary. Try it and post your results in the comments.