Hello,
I have an FP2 (bought in December 2018) and I am facing the same issue, that is, the screen freezes when making or receiving call (not always).
Since that thread is closed and no solution is posted there, I am opening a new one.
In addition to that issue, the screen also always freezes when the alarm app goes off. It also freezes once in a while randomly while browsing and while using the messaging app.
The issues seem related but I am not sure they are the same. For me, the UI itself freezes, and I am unable to close the app (browser, Phone app). The phone becomes very slow and unresponsive. After a while, a message saying UI (or app) is not responding pops up and asks me to either close the UI (or app) or wait. Even when I click on close (this takes time as the screen is not responsive), the phone does not get back to normal. I have to reboot for the phone to behave normally. This means I need to reboot regularly for the phone to function.
For you, it seems you are able to close the app without any problem and just doing this seems to work.
Other than the main issue above, I also have a minor issue with my phone. Once in a while, some parts of the screen become pixelated. This lasts for a few seconds after which the screen becomes normal. This issue does not impact my daily use of the phone but can be related to the main issue.