My case - this issue was caused by non-original - new front cover (display). Purchased over E-bay from China. The proximity sensor (in left top corner) detected the object imediately after the call was estabilished. The bottom area of the cover is black painted and it was sligtly painted over that proximity sensor window as well (you chave to see through by eye). Solution - take a sharp knife and carrefully remove that black film. Problem solved :-). I can immagine a foulness in theat area will cause the same issue with orriginal Sony cover.
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