Why my LG E400 screen goes off during calls?

[AC Question]

All of a sudden the screen began to go turn off during calls. It comes back once the conversation is over (once other side has ended it, I have no control over conversation while it is active!). I recently gave the phone to a relative as backup phone, and she may have pressed some setting unintentionally, but I havent found any setting that would seem to be resposible for that. Ever since then I have reseted the phone to factory settings, updated the software but to no avail.