You are right NextG, I had not thought about recharges. I have installed the TrueiService app and am still investigating it. Maybe it's best to stick with True. Thanks for your help NextG.
In case it helps anyone, I have asked in the True shop what to do when the advertising messages arrive. They tell me that what they do when it happens to someone is dial *137 and the call button. And once inside the call, press the number 4 several times. They have not told me exactly how many times, but they tell me that they do it exactly like this and all advertising messages are cancelled. In case it helps anyone...