Help! Why Are My Incoming Calls Being Blocked?

As the screenshot shows some incoming calls apparently at random are being blocked. I get no notification in the lock screen or notification window that the call was made - it will just appear on the phone log.

I have no numbers on the phone block list nor do i use any third party blocking app.

What usually happens is the phone will ring once or twice before it decides to "block" that particular call. Some of the contacts are on my contacts list.

I have the international unlocked version.

Thanks for any assistance!