I've used to have same issue using the AirPods Pro. Hence I avoided opening / closing the AirPods Pro Case while on the call. However, recently while on the call, I had to open the Case to take the second bud and then close the Case, and the call did not drop. So the issue no longer persists, at least on the iOS 14.2 upgrade.
Perhaps, try calling a number like a bank or govt service that requires to pre-select from a menu first so that you have ample time to test how the AirPods are behaving without inconveniencing a real person on another line, and see whether disconnection occurs again.
In the Bluetooth menu for the AirPods there does not seem to be any specific section dealing with iPhone's reaction to opening / closing of the AirPods Case. Just in case, my present Settings>Bluetooth>AirPods Pro>(i) are:
Press and hold left: Siri
Press and hold right: noise control
Automatic ear detection: yes
Connect to this phone: automatically
Spatial audio: yes
Microphone: automatic
Optimize battery charging: yes
Hope this helps )