Thumbs up People hear echo during phone calls

Hello everyone
I just bought a Galaxy S4 Active and people started saying that they hear an echo of their voice every time they call me or I call them.
I tried different things. I noticed that every time I remove the back cover the problem is gone, then I put it back again and the echo is back. I unchecked the noise reduction option but nothing happened.
Does anyone have a solution for this?