I’m in the uk on the 3 network, having purchased an iPhone X I’m finding exactly the same problems with audio on phone calls while using the handset earpiece as previously mentioned by fellow users of the 7, 8 and X series iPhones.
When I had my IPhone 6+ I would talk frequently with a friend who was using a 7+ many a time his voice would fade away unless he spoke directly into the phone, he on the other hand had no difficulty hearing me while both of us were using the earpiece.
This problem to my thinking is a hardware issue, mic, earpiece or both because if it could be rectified by a software update, don’t you think it would have been addressed by now - I do.
These devices are incapable of performing their most basic function - phone calls without issues, they are even supposed to be VOLTE capable, you would never think so, it’s like taking a step back in time from digital to analogue audio.
The best audio seems to be available when you are using your 7,8 or X series handset with a car system or Bluetooth device, AirPods for example although even those have issues such as wind and background noise because they do not have a noise cancelling system.
Ill be returning this, my second iPhone X for a full refund, phone and case and reverting to an older model until I can find a suitable alternative having become increasingly dissatisfied with the brands recent compromised offerings.