The consensus is that it's "pay-as-you-go," but, wow, this is *not* a service that's available in the U.S., like in the rest of the world. Every provider insists that you pay a base monthly fee, which strikes me as strong evidence that U.S. mobile companies are an oligopoly.
I just want a SIM so I can have my network failover to a cellular router when my Brightspeed connection goes down (as it does a few times a week now, though I hope it'll be less often when I have gigabit fiber shortly).