If the new iPhone is unlocked and free from network restriction, you can definitely use it with your Tmobile SIM card. If the iPhone is restricted to some other specific network, you need to unlock it in prior. You can unlock your iPhone using remote unlocking service. You can get the remote unlocking service for an iPhone from MobileUnlockSolutions.com This method of unlocking an iPhone is safe, simple, affordable, quick and permanent. It doesn't require a jailbreak too. Good luck.