Most of the phones that have been previously unlocked will work in Tanzania. It is to be noted that Tanzania uses GSM 900 and 1800 while USA uses GSM 850 and 1900. It is possible that your phone may not possess the correct frequency bands to work properly in Tanzania. Check the specifications on your phone before you travel. In the worst case scenario, you could buy a cool smartphone which is offered at cheaper prices by operators like Vodafone and Airtel