“Is there any contingency plan, that you know of or suspect, that would immediately disconnect service to all “welfare phones” as though the bill hadn’t been paid?”
No. Those phones are locally purchased without planning of any sort. Of course a list of those phones is available and they could one by one turn them off, but there isn’t a wholesale capability.
Maybe not at the moment, but it would not take long for the cell-provider's software people to create one.
Then again, I would think they would have an app to turn off all phones belonging to a specified bill payer, and the thing that the welfare phones have in common is that they have a single bill payer (the government entity administering the program).