My phone showing calls from another phone on my account.

I have an iPhone 5. There are four other phones on my account. Phone calls from one of the phones (iPhone 6) are showing on my call log. Today (for the first time) I even got a notification on my lock screen that I had 3 missed calls from someone actually calling the other phone. (It did not actually ring my phone or leave an entry in my call history this one time!)

I checked settings to make sure I had not selected this other phone number, but that particular phone number did not even appear on the list! That one phone is the only one leaving records of its calls in my call history. Since it is the only phone not listed under the iMessage setting "You can be reached by iMessage at...", I feel like this might be a possible connection to the problem.

Any ideas? Thanks!