While I don't use wi-fi calling, a couple of my friends do and have not experienced any issues. This excerpt is from this support document, Make a call with Wi-Fi Calling - Apple Support
Get help
If you can't turn on or use Wi-Fi Calling, make sure that your carrier offers Wi-Fi Calling and that your devices have the latest software. Then, follow these steps if you can't turn on Wi-Fi Calling or make a Wi-Fi call. Wait two minutes before you go from one step to the next.
- Go to Settings > Phone > Wi-Fi Calling and make sure that Wi-Fi Calling is on.
- Restart your iPhone.
- Connect to a different Wi-Fi network. Not all Wi-Fi networks work with Wi-Fi Calling.
- Turn Wi-Fi Calling off and then on again.
- Go to Settings > General > Reset and tap Reset Network Settings.
If you believe it to be a software problem with the iPhone, ensure that you have the latest iOS update on the device and work from there. You have the option to restore the device to factory and set it up as new, and then test the wi-fi calling without using your backup and see if it makes a difference. If it does, that could indicate a problem with something currently on your iPhone that is corrupting the backup. Up to you how you wish to proceed at this point, however to be honest, the only thing the iPhone does as far as wi-fi calling is concerned is to provide the portal to the carrier's system.
