If the Ring app on the iPhone is open (i.e. foreground) and a phone call comes in, the ringer is suppressed. Unless you are actively looking at the phone and see the banner notification, you will miss the incoming call. Maybe this is so common with IOS that it doesn't surprise other people, but it's been a nuisance for me and took a while to figure out. I would be in Ring, then put the phone down, and wonder why I had voicemail when I was sitting right there when the call came in. (I verified that incoming calls rang audibly as expected if I did not have the Ring app in the foreground.) Just posting this here in case anyone is experiencing the same thing and searching whether it is a known problem.