[DAD-PC]

even after unlocked on att, you will either need the phone activated on sprint, or an active ATT sim card to place calls. borrow one of your friends SIMS, pop it in the phone, and see what happens.

Note that on the GSM side, it will only be 2G, as it lacks the American GSM 3G bands for ATT and Tmobile.
And it takes a little bit of editing to get the internet to work, though it should make calls and send SMS's just fine.