Why does my iPhone 8 Plus ring through speakers when headphone is plugged in?

My iPhone 8 Plus is using the latest iOS 11. When my earbuds (corded) are plugged in using the connector that came with the phone and I get a phone call, the phone rings through my phone speakers, not my earbuds. Why is this? I thought when the earbuds are plugged in, the phone would lower whatever I'm listening to and ring through my headset. I works like my phone to ring through my headset when the headset is plugged in. I would also like my phone to ring through my phone speakers when my headset isn't plugged in. Please help.