Does it just switch off automatically? Or does an error pop up? Did this happen before or is this a new issue that has popped up recently 

Seems to switch off every time I go out of range of my wifi. No error code.  It only started a couple weeks ago, before that time it didn’t do that. When it switches off, you have to get a new access code, real pain as my cell signal is very weak.

Can you reproduce the error 100% of the time you leave the house and WiFi? 

Yes, every time I leave my home wifi, I have to get a new access code to sign back into wifi calling. It worked perfectly for a month, then all of a sudden about two weeks ago, I started to have to turn wifi calling on and get a new access code, every time I leave my home wifi range.

Do you have another phone (family member or friend) you can pop your sim in and see if the wifi calling issue follows your sim to the other phone?

You can also try wifi calling with your friend's sim in your phone to see if the issue follows the phone