iPhone XR isn’t connecting to headphone mic. How do I change the built in mic default to headphones?

when I plug in My Apple headphones to my iPhone XR uses the phone microphone rather than the mic on the headphones. So if I put the phone in my pocket people on the other end can’t hear me. I’ve tried reseeding the connection and tried several Apple headphones - no luck. I tried changing my settings - turning off the feature that auto selects the speakerphone when the phone is taken from my ear - no luck. I searched for the Call Audi Routing setting bc I read that worked but could not find that in the General or Accessibility sections of the Settings App of my iPhone XR.