This definitely does not work on my A T and T G3. Not sure if it is carrier specific but all I get when I do that is a normal boot. Have tried it several times. Very frustrating as I would like to wipe the cache.
Hold the power and volume down for as long as it takes until you see the recovery menu. I have to do this with my Verizon G3 because getting to that menu is slightly different for some reason.