Probably because screen stays on during a call and you do all sort of crazy things with your ears.
Calibrating proximity sensor from my experience does very little to help with this problem.
What @MrCrayon suggested is probably the best solution
So the problem is that it doesn't turn off the screen and touching your face to the display triggers what you don't want.
The proximity sensor of our phone has a very narrow detection area. The space is located in the area below the top speaker outputs (top left). The ear should preferably be attached at this point during a call. So not in the middle of the top edge of the phone as usual. It is definitely uncomfortable. In addition, the sensor is less responsive if the phone is not in a vertical position. The proximity sensor is probably the biggest weakness of this phone.
The best way to check its function is to use the AndroSensor app.