They’ll try, but cell phones, like computers, always seek out “work arounds.” For example, during riots and natural disasters, to cut down on circuit overloading, the local phone prefixes can be blocked. But if you have a phone with outside prefixes or another area code, they will still work. IOW, cell phones will still be in play. (This is a good reason to carry a cell phone with an out of state area code. It will still work during a disaster or other crisis.)
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?