Hi if the new iphone is unlocked one then you can freely use it with t-mobile or any other GSM sim .Else if the iphone is locked to some other network provider then until you unlock it you can't use it with t-mobile sim .You can easily unlock your iphone through providers like Unlock-Zone.com they are providing remote unlocking service to unlock all model iphone from network lock ..After purchasing the service visit sites like MobileUnlockGuide.com to get free instruction about how to use that service and unlock your iphone .After unlocking you can freely use it with any GSM sim ...