Can't hear the other party during whatsapp or Line call but phone call works fine

[AC Question]

Hi,
I;m unable to hear on whatsapp call. I thought it's a whatsapp problem but when I tried on Line, it's the same problem. The other party can hear me but I can't hear them. all other phone functions, whatsapp function seem fine. I can make phone calls via whatsapp and Line but i'm unable to hear the other party. Please help!

I'm using Samsung galaxy A5.