I made this comment on another thread that ended up getting flagged as dupe, but this is probably not all that mysterious. These SIM boxes are a commercial product you can buy from China:

https://cnetross.en.made-in-china.com/product/OSomfpPGJWUH/C...

...and their purpose is mostly to provide an IP-to-cell-phone-number gateway for SMS spam and phone scams. A real cell phone number is greatly preferable to VoIP phone numbers, which are blocked / flagged at a much higher rate.