Samsung S5 developed sound problem when I call. Help!

A

AC Question

I have samsung S5. Last week it has developed a problem: when I make/receive calls I can't hear another person and they can't hear me too!
Everything works fine when I use the Loudspeaker during the call.
Apparently the problem is with the software, not the hardware, beacsue when I call via skype I don't have any issues (microphone and build-in speaker absolutely work fine)

I know that I can reset to factory settings, but afraid to loose all the data . Is there any other way to repair the OS/fix "Call" application?