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 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

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