Just had my daughter's iPhone 7 Plus with the same issue fixed. We noticed the problem after she started using Air Pods.
It was easy to fix it, we did a hardware reset, on the 7 Plus you have to press the power button and volume down simultaneously, power button first. Than you hold both pressed, the phone will turn it off and as soon as you see the apple logo you can release.
We did it and issue was fixed, it looks its a glitch between 7 Plus and Air Pod 1st gen.
Good luck!