Depends. All Sprint, Verizon, U.S. Cellular, and Alltel phones will NOT work on t-mobile. T-Mobile uses 1900mhz gsm (and 850mhz roaming). The phone has to be unlocked (if from At&t or from a different provider) and support at least 1900mhz. (and 850mhz for better roaming). Just insert sim. That's all!
Any old T-Mobile phone can be used with T-Mobile. Just put in the sim card! No calling them is needed.