My proximity sensor doesn't work after call waiting

My proximity sensor works fine during calls. Until someone calls in while I'm on the call. Whether I answer or not, when I pull it away from my face it activates the screen as it should, but it never turns it back off when I continue the conversation. This ONLY happens with call waiting. once i press on the home button and unlock the phone it works fine, knowing that i had a replacement for the phone for the same fault and now and i don't know what to do as my brother's phone is exactly the same however it works fine.

I have an iPhone 6 running 8.2