[Q] Accidentally putting calls on hold

I love the Note 5, but I keep accidentally putting calls on hold when using it in the "traditional" way - putting the phone to my ear.

I've discovered that somehow the screen is coming back to life while I talk - and I'm accidentally touching the "On Hold" virtual button on the call screen in the upper right corner.

I feel a bit stupid even posting this, but the Note 5 is my first Lollipop and phablet experience. I don't remember the "On Hold" button even showing up on my GSIV call screen. Could it be a proximity sensor issue?

Has anyone else experienced this and if so, is there a fix?