Cannot make a phone call... retry

I posted this one earlier and it was merged with another topic (nl. to wait 5 secs before selecting a sim-card). My problem is different: the app does not respond at all when selecting a name or number. So, I do not even get to the point where I can select a sim card. Here’s my original post. Still looking for help, because it keeps being very annoying.

Of late, increasingly often, I cannot make calls with my FP2.
When I select a contact, or a number, simply nothing happens. Not the popup to select a sim-card. No error message. Just nothing.
Same when I key in the number myself: after pressing the call-icon, again, nothing happens.
When I reboot, I can call again, but after a number of calls, the misery starts again.

Anyone any clue? Since I want to use my phone mostly to make calls (I’m quite oldfashioned in this respect, maybe) this is of course terribly annoying…

Help appreciated!

Did you already update to Android version 6? Does the problem persist there?

Yes I did. Since then, my phone kept crashing. Support told me to go back to factory settings. Now for two days trying to get my apps, settings etc back… Looks like this call problem is solved - new problems appeared, but that’s for another thread :-/