My postings have never been on or about T-Mobile WiFi calling, so I apologize if they seemed to be. They are one of the carriers the MVNO US Mobile and its’ plans that are based on, along with Verizon service. Those plans are called “Warp 5G”. Currently they offer 3 Warp plans that utilize the VZ US carrier that have foreign roaming. They are called “Unlimited Starter, Unlimited Premium, and Shared Data”.

I am a fan of the IMS (Internet Protocol Multimedia Subsystem) function on dual sim phones that allow it. I use my T-Mobile service for the data line component of that, and perhaps that discussion is where you were confused. It has been discussed here on FT for the past 2 years or so. T-Mobile only provides the data service that is necessary for IMS capability. As I mentioned, I have several TM lines available with their included free international data roaming, and use it every trip. A search on FT or the web would probably clear up many of any questions you may have.

Roaming data is data, it doesn’t matter if it’s Airalo or T-Mobile. It is only the medium within which IMS functions. You probably didn’t mention how their roaming data was being used to your friend, or they are ignorant of it’s capabilities also.