The system as a whole has to know how many minutes the phone has. Thus in one or the other manner there is a central point of contact.
That's true, and that is probably the only way you would be able to go about it. Whether or not they could dump them all at once would depend on the ESN's of the phones they wanted to shut down all being in sequential order or all within a certain range of numbers. Otherwise, it would be one at a time, unless you just shut everyone down regardless.