FP2 Proximity sensor is in top-left corner, and cannot detect an object placed centrally over the top of the phone, or on the top-right corner of the phone.
As a result, left-handed use, where the ear tends to the top-right of the phone due to natural orientation of use, is rarely detected by the proximity sensor.
I have confirmed using the sensor readout app that the proximity sensor is working, and exhibits very specific locality to the top-left corner of the display.
The only work-around for left-handed use seems to be to install an in-call screen-lock app, so that you can’t accidentally touch the phone with ear/cheek and have it do all sorts of things you don’t want it to.
If I remember right, all my phones so far had the proximity sensor at the left upper side.
Since I am a “lefty” myself, I almost exclusively use my phones at my left ear but never had any problems, same with my FP2.
Maybe my ears have grown bigger with time
The little app WaveUp has a feature to vibrate when sth. comes near the proximity Sensor. You could use it to accommodate your phone calling physics to the sensor location.
But the workaround described by @TobiasF is simple and convenient.
