Having same issue. Xifnintiy sent new SIM card. No help. Spent over an hour on phone with Apple support. Although the level 2 representative insisted that these Wi-Fi calling issues alwayhs are the responsibility of the carrier, yhey did a diagnostic download and just got back with me with the results. They confirmed that Apple engineers indicated that this is "an issue on both sides and are working with several carriers to resolve". The representative indicated that when resolved, it will be part of an update to IOS and they are going to email me when that occurs. So, I guess I have no choice other than to return to AT&T which I just left to switch to Xfinity.
I also brought my own device (Apple X) which I purchased from an Apple Store and confirmed compatability with Xifnity Mobile before switching. Of course Xfinity tried to imply that because I didn't purchase the device from them they weren't responsible. One Xfinity representative even went so far to say that the issue was in my Wi-Fi network (which happens also to be Xfinity--go figure). My wife has the same model iPhone (Apple X) also purchased from an Apple Store, but running on Verizon. Wi-Fi calling works fine on her device and on the same Wi-Fi network. Hard to figure this out since Xfinity Mobile uses Verizon's celluar network. Seems here is definitly an incompatiability going on between Xfinity Mobile and Apple.
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