Headphones not a choice for phone calls?

My wired headphones with lightning adapter won't work with phone calls on my iPhone 12. I am guessing it is a simple fix, as the headphones work fine with YouTube and other apps, just not with phone calls or voicemails. I have powered down & restarted, looked in Settings, and pulled down the upper-right menu which gives me choices of speaker and other devices in my home - but I am not finding a way to direct the phone to the headphones. Anyone know the fix for this?