It is a feature that Apple built in to iOS that allows dual sim phones to simulate WiFi with two lines. The basics are, line 1 connects with a local carrier as your data line. Your line 2 chooses a carrier in Network Selection that has no roaming service. It then defaults to line 1 as if it were a WiFi connection. WiFi needs to be turned off for this. So, if you use line 1 on a paid data service like Vodaphone or Three UK and have a data connection, your main number T-Mobile line 2 with no service will get free WiFi calling to the US. This works well and is easy to set up. Your main line remains active as long as this setup remains. It is very stable and you can leave your everyday TM cell number on for incoming calls all day.
I use either Airalo or sometimes just another idle T-Mobile line on my account in the data position with the free 256k data roaming.