This is known problem on both Android 5.1 and Android 6.
You need to recalibrate the proximity sensor. See here how to do that:
https://forum.fairphone.com/t/proximity-sensor-tool-or-how-to-fix-the-proximity-sensor-issues/21411?source_topic_id=29029
Note that you need to have the latest Android version installed (on Android 5.1 FPOS 1.7.0 or later, on Android 6 FPOS 17.05.02).
Btw: There is search function in the forum; you can find it in the top right corner. Many issues have already been discussed, so its always helpful to fist search for an issue.