I'm considering an iPhone 3GS to buy (to hold me over until the iPhone 5), but I currently have service with T-Mobile. The owner said he updated the iOS to the lastest version (4.3.3) when he restored it. Is whatever baseband comes with 4.3.3 upgradeable to 6.15.00 and then ultrasn0wable, like they have been in the past? Thanks for your help.