I realize this thread is 4 years old. But it would seem that Apple hasn't fixed this issue. Or it has come back. I do recall on my old 4S and 5S, that when my earbuds are plugged in, the incoming call ringer doesn't sound through the speakers, only the headset. That's pretty much any audio. Like an automatic mute once the headphones are plugged in. I now have the 6S, and even though my buds are plugged in, the incoming call ringtone goes through both the speaker and headphones. Very annoying especially when your at work or in a quiet public area. Yes, you can turn the mute switch on. But you can forget it's on. So when you have the headphones unplugged, and you get a call while your in another room, you won't hear it.
Not sure what genius decided to do it this way. How hard can it be to make the iPhone transfer ALL sounds to the headphones when it's plugged in. And through the speakers when it's not. Or at the very least give an option in settings to hear (or not hear) ringtones when headphones are plugged in. Oh Apple/Cook. Your messing up what Jobs made so well.