thank you, you are right and i agree with you but i checked websites of at&t and t-mobile. they have lists of compatible phone names and informations but they don't mention " xperia go" so I'm not sure yet, does it work in us or not!
at&t Has emphasized, phones must be:
Operates on GSM network
Operates on 850/1900 MHz bands
Unlocked - not programmed to work exclusively on a wireless network other than AT&T
and my " xperia go" is precisely this but its name isn't in the list of compatible phone names.