WiFi calling not working in iPhones

I moved my whole family to Smarty a few months ago, and calls failed for all of us seemingly randomly - until we realized that it was whenever the call was trying to be made or received over WiFi (we all had WiFi calling enabled, as reception in some areas at home/work isn't great). Never had an issue before with different carriers, and our iPhones are all clearly marked as supporting WiFi calling on Smarty (per https://help.smarty.co.uk/en/articles/4237414-which-devices-are-compatible-with-wifi-calling).

We've spent hours on the phone with Smarty, and received zero valuable help (we were told to reboot / reset our phones, and even went as far as completely wiping and resetting some of the phones). One of us even got a new phone iPhone, and it didn't solve the WiFi calling issue. The carrier profile is "Three 66.0", so it seems up-to-date too.

Anyone else having issues on iPhones? Smarty support seems completely out of their depth - the reps I spoke to could only repeat that we should reset our network settings repeatedly, blamed the problem on Apple, and weren't even aware that some of the settings were set through their carrier profile file...