iPhone Rings but Screen is Blank AND...

RESOLVED! (Not a phone call problem at all!)

It was a configuration problem having to do with Messages. Here is how I had things configured.

1. 1. Same sound used for both Ringtone and Text Tone
   2. In Settings > Notifications > Messages the Alerts for Lock Screen and Badges were turned off.

Because of (1.) when a Message was received, it "sounded like" a phone call was coming in.

Because Message Alerts to the Lock Screen were disabled in (2.) the Lock Screen remained blank/black when the presupposed call was received - which was very mysterious. Equally mysterious was that the Phone badge did not increment when these presupposed calls were received.

Because of (2.), Message Badges were not displayed. They therefore were not incremented whenever the presupposed phone calls came in. If they had been incremented whenever these presupposed phone calls came in - and if I had noticed it - diagnosing the problem would have been faster and easier.

Also, as part of the diagnosis process, I looked at Recents (from the phone icon) to see who the call was from that didn't wake up the lock screen. Because there was no phone call, none was found in Recents.

SOLUTION

1. Set separate sounds for Ringtones and Text Tones.
2. In Settings > Notifications > Messages toggle ON Lock Screen, Notification Center, Banners., and Badges.

For good measure I gave each app its own sound.

Thanks for the reply. I will keep looking for a pattern. As to your initial suggestions...

1. The calls don't get logged in Recents. (Is there another way to get caller information?)
2. Ditto.
3. I am on my home network 99% of the time.
4. Focus is a possibility. I will investigate. Thank you.