Generally speaking, to use a Cingular phone on a different network, you need to get an unlock code from AT&T. If AT&T won't cooperate, some third parties claim to know the algorithm for calculating the unlock code and will sell it to you. Buy at your own risk. Please note that most phone only allow a certain small number of unlock attempts before they disable unlocking, so you may have as few as 1 or 0 attempts left on your 650.
Once the phone is unlocked, if you insert a T-Mobile SIM, it should just work. I used an unlocked Cingular 650 with a T-Mobile SIM in the U.S., just as a test, then used it with a French SIM when I was in Europe. No problem.
(Getting the unlock code was the biggest challenge. Fortunately, the phone was a hand-me-down from a current AT&T customer, so they were cooperative, although they confused us by first giving us the code for my friend's 680...)
