https://www.aliexpress.com/item/32838878454.html
I just got one, and it works great.
Functionality:
Imagine it like a remote-controllable mobile phone.
1) connect box to your home internet with a LAN cable
2) install app on your phone
3) bind app to box
4) stick SIM into the box
If a call or SMS is received on the SIM in the box, the call/sms is transferred through the internet to the app on your phone. The phone thus needs data, but it doesn't matter where the phone is.
So no matter where in the world you are, you can suddenly make and receive calls/sms throughout the whole world(!) without paying roaming charges.
The app on the phone can be connected to multiple SIM Boxes, so you might as well have a foreign SIM card sitting in a box somewhere else, and use the foreign card from somewhere else. Imagine placing a second box at a friend's place in a remote country and use it from here.
Quite some scenarios that this could come in handy for.
Drawbacks I found so far:
1) No USSD codes can be run on the box.
2) Manual network select is not possible, so if you're close to the border the box might log into the "wrong" network and clock up roaming costs.