Before i reported this issue, I had recently (beginning of October) completely reinstalled FP Open OS (to get rid of google apps), and it didn’t change anything. If anything, the hangs got more frequent.
I just found some people describing a very similar issue: FP2 incoming call screen freeze issue People there report that the phone app seems to hang a long time.
One more peculiar thing I noticed is that when this freeze happens, the background image is reset to the “default” gray fairphone image, instead of my custom one; no other settings seem to be affected though, and it makes no difference whether my custom background image is stored on the internal storage or on the SD card.