Iphone on Wifi calling not rining when in apps or doing something on my phone

hello everyone.

When at home I use Wifi calling and up until recently it has worked fine however since the last update I have been having issues.

When my phone is in the lock screen it works and my phone rings and I am able to answer it as normal.

When my phone is not on the lock screen and I am in an app or doing something it wont ring it just cuts the call off, it does not go to answerphone or anything.

When I take wifi calling off it works fine and as I would expect does anyone know why, do I need to change a setting to get it to work when the phone is being used.