If you have LineageOS 7.1.2 (and have root), try this solution from XDA.
After having tried all the solutions proposed here, none of which worked for my Nexus 4 (mako), I found one on XDA that solves the problem with the Android dialer (but not with other apps). Basically I downloaded a recompiled version of the Dialer.apk file, which simply ignores the proximity sensor and behaves in the same way as the stock dialer app does.
Rename /system/priv-app/Dialer/Dialer.apk to something, then place the downloaded file to that folder. After reboot, I had to install the new dialer manually (simply by clicking on it). So now the original app is replaced, and the calls should be handled by this new one.
[Downside: the new way to answer a call is by pulling down the status bar and clicking 'Answer' (or 'Dismiss'), the usual slider is missing. Also, you'll need to repeat this every time your Android updates to a newer version.]