SIM farms are another possible explanation. FBI just busted one with hundreds of thousands of SIMs just a few weeks ago.
edit: Actually this is what I'm getting increasingly angry about: providers and platforms not doing anything against bots or low value stuff (think Amazon dropshippers too) because any usage of their service, bots or otherwise, are metrics going up and metrics going brrt means profit and shareholder interest.
But yes, they also might not care if they are getting paid. If the SIMs are only being used for voice/text as I suspect, it might have very minimal load on the network.