[1] https://www.pbs.org/newshour/nation/how-sim-farms-like-the-o...
a) Doing some weird grey market VoIP thing. 32-in-1 GSM to SIP gateways have been a thing for a very long time in the developing world. Maybe they think they found some arbitrage route for phone traffic to/from the US PSTN that they can profit from. Anyone who interacts with grey market voip stuff will recognize these things immediately.
b) Using them for something like receiving 2FA authentication codes to create bot/socketpuppet social media accounts. In this sort of scenario they'd have live phone numbers/service and the cheapest possible phone plan, and ability to receive incoming SMS. The accounts then get provided to some other group of people who are doing mass advertising/social media manipulation.
The OP ones are actively scanning the vicinity and acting like BTS to connect to phones automatically, equipped with radio antennas, SDR, etc. to gather the victims numbers in real time and send them spam/phishing while the phones are connected to to these BTS
The real story is the government didn’t really care about users being spammed, you get those all the times and there’s little regulation to protect you (like preventing corporate from selling your number etc.), they cared because with these devices people can and will communicate outside of the approved channels, that also might be encrypted too, so harsh charges and make it as public as possible to deter others from doing the same, even if they were not in it to scam or phish people, and notice on the emphasis on “blocking the 911 calls!!” so jamming charges are there too.