Headphones work for music, but not for phone calls

Using iPhone X, apple headphones with 3.5mm jack & a 3.5mm to lightning adapter.

Headphones work fine when playing podcasts or music through the adapter and headphones.

Phone calls will now only work through speaker mode or held up to ear - no sound through headphones.

Have tried checking connections - all firmly connected. Have tried a soft reset. Have tried multiple sets of headphones and different 3.5mm to lightning adapters.