It's likely the person making the calls bought it from the person who stole it. Why else would they be making calls? The first thing I'd do if I stole an iPhone would be to remove the SIM, go into AT&T and say I bought this off Ebay, give me a new SIM....or jailbrake it, but for sure remove the SIM immediately.
Plus if they could do all that, they are probably reading this thread!