Earphones do not work for call

I have recently acquired a refurbished iphone X 256Gb running IOS 13.3.1. It is great apart from one problem. As it does not have an earphone jack I purchased an adaptor. This works great for playing music etc, but when I receive or make a call, it does not come through the earphones, but stays with the phone.

I have tried several different earphones and adaptors - same result. I have tried taking it out of the case, but do difference. I tried a settings reset, but no difference.

The phone is still under the refurbisher's warranty. Is it faulty, or is this a setting? Any help gratefully appreciated.