WiFi calling drops calls on Iphone shortly after connecting

[Big-Gee]

Update..... 

I have the option to switch the router from the default MODE 1 to MODE 2 or 3 which has some (unspecified) effect on band steering and channel handling, so I have tried switching to mode 2 to observe whether it fixes it and whether there are any noticeable issues with existing performance and update this as it progresses 

[licquorice]

https://www.bt.com/help/broadband/what-is-the-wireless-mode-feature-on-my-bt-smart-hub-

Explains what the modes do.