Proximity sensor problem during calls

I have a recurring problem with things on the screen being activated during my phone calls; the mute button going on (very, very common), calling someone else, setting up a new SIM card and profile (I don’t even know how to do this!!) etc. When I have tested it the screen does seem to go off when I hold the phone up to my ear, but during the conversation it’s obviously coming on again without me realising. Also, when I put the phone down having finished it often stays off and I have to press a button to reactivate it so that I can end the call.

How can I stop this happening - it’s very disruptive!!

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