The short answer is no. Most phones that you buy directly from a cell phone provider are "locked" with that provider. So even though both Cingular and T-Mobile are GSM carriers, their SIM cards would not be interchangeable if the phones are locked. However, you can buy unlocked phones either directly from the cell phone manufacturer or from abroad. With unlocked GSM phones, you can place any SIM card in it, and it will work. The phones will be more expensive, of course, as they will not have any carrier backing. Alternatively, you can also unlock an existing locked phone, but you'll have to research how to do so or find a local business that will do it for you. Note that unlocking phones is not illegal by any means, even though the providers may not like your doing so.