Lightning to headphone adapter does not work for phone calls
Like others in the community, some headphone adapters do not work for my phone calls but work fine for any other audio, e.g., podcasts, music and so on. Apparently to work for phone calls you need an " MFi Certified Lightning to 3.5 mm Earphone Jack," which has a small integrated circuit in the lightning side of the adapter.
Note: I have not tested this yet. I'm buying a MFI certified one to try. Your mileage may vary. Try searching for "MFi Certified Lightning to headphone," instead of just headphone adapter when looking for one.
iPhone SE