Why does HD voice go away when receiving a 2nd call when on an existing call?

A

AC Question

I have an s5 and my girlfriend has an s6 and whenever we are on the phone and one of us gets a second call the other person can tell and the call quality changes drastically. Whether you answer that second call or not we cannot reclaim the original call quality and must disconnect and call back to have the best call quality.

We think it only happens when we are on our headphones but nonetheless I am searching for answers and so far it seems no one else is having this issue. Her s6 is brand new...any ideas?