1) I think the above code needs to be 004 instead of 002. I tried 002 and it forwarded a normal call to google vm. 004 is for all conditionals (ie IF busy, IF, unreachable,...) and 002 is for "all 4" which seems to be just all the time.
2) If someone calls your cell phone and you're busy or don't answer, then they get redirected to google voice, google voice will then try to call your cell phone if you have that configured, so that's one hell of a confusing round trip for your callers.
To return to AT&T VM, you'll need the phonenumber for that, which should be possible to obtain with #002#, then re-set with 002*(AT&T VM number)#