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.