Hi @Mapoljo, let me see if I can help out here!
The proximity sensor is there in order to detect if your ear is within proximity or not, and if it is then your display will go dark during calls. This is to avoid you accidentally ending calls with your cheek for example. There is no way to deactivate this sensor using the device settings, the only time the proximity sensor is deactivated is when a headset is connected.
I hope that answered your question!
