FaceTime call disconnecting Wi-Fi.

Hello Jujubees_,  

Welcome to the Apple Support Communities.  

Do you currently have Bluetooth and Wi-Fi calling enabled for your device? If so, test using FaceTime with both features disabled for now.

• Set up and use Bluetooth accessories on iPhone - Apple Support
• Make a call with Wi-Fi Calling - Apple Support

It may also help to test FaceTime with LTE disabled temporarily: About the LTE options on your iPhone - Apple Support

Please also be sure to update and restart your device before testing again:

• Update your iPhone or iPad.
• Restart your iPhone.

All the best!

I’ve had this problem with Facetime for years, and there has never been a fix or a straight solution as far as I know

I just turn off mobile data. That way the phone loses the option to switch networks, because there is no other network, so it is forced to use wifi