I googled this just a few minutes ago and though no one helped here, I found the answer and wanted to help.
I had to set my lock screen to swipe only, then the option for the phone button was there above the camera button check box. I unchecked the box, then changed back to a pattern lock screen (and went through the settings for that).
My 1 yr old grabs my phone whenever he can, which is why I have the pattern lock. But with the phone button, he was managing to call my mom from the emergency call screen when we woke up at 5:30am while I got him breakfast.