Out of curiosity, on your new carrier, can you still do WiFi Calling?
Back in the WP 8.1 days, WiFi Calling was a separate app on T-Mobile branded phones, since they were one of the first carriers to offer it, so they had to develop their own app. So if you wanted WiFi Calling on T-Mobile you HAD to buy a T-Mobile branded phone. (And of course the WP selection on T-Mobile has always been very poor; lately only one model at a time.) When WP 10 came out, I understand it has a built-in generic WiFi Calling app, which will NOT work on T-Mobile. Does this mean the T-Mobile IDOL 4s still has a separate WiFi Calling app that works on T-Mobile? If so, what happened when you took your T-Mobile branded IDOL 4s to another carrier? Do you have both a T-Mobile WiFi Calling app and the built-in generic WiFi Calling, and so you now use the generic WP 10 WiFi Calling app on your new carrier?
(Sorry for any confusion; I have never really seen WP 10 yet for myself, but I wanted to buy a Lumia 650 last year, and the only option was an unlocked one, but I was told that even though WP 10 has a built-in generic WiFi Calling app, it would NOT work on T-Mobile... and so I couldn't buy it, because I need WiFi Calling, and I'm on T-Mobile.)