Phone Starting To Face Dial topic

So I don't get why this has started to happen and am hoping someone can help.

My phone has started to face dial while I'm on a call. The screen is off... so I have no idea why it's doing this... and it didn't do it when I first got the phone - it just started about a week ago.

I am rooted and running a "custom" rom - it's the debloated OTA (12B). But otherwise it's a "stock" phone.

Any help much appreciated. Thanks!