Missed calls notifications
When I look at my missed calls on my iphone it immediately calls the number before I can check who it is from or if there is a few missed calls, it calls the last number back.
iPhone 6, iOS 12
There is a setting on my 6S in Settings, TouchID and passcode, which says "Return Missed Calls" that I turn off.
The iphone 6 ios12 may have a different wording but it is probably in the Passcode settings area