Ya sure you can switch your t-mobile sim in to your new iphone 3g and use it freely without any issues only if the iphone 3g is unlocked one .Else if the iphone 3g is locked to some other network provider in other countries then you can't use it with t-mobile sim until you unlock it .You can easily unlock your iphone 3g through providers like Unlocking4U.com they are rendering remote unlocking service to unlock all model iphone from network lock .You first verify that whether the service is available for your country and network provider .If yes then pay for it and unlock your iphone 3g ...