I had the same problem with my new iPhone XR.
Looks like its the cover I bought to protect the phone that is causing this issue.
I had a foldaround cover to protect the back and the screen on my previous iPhone (6)
It worked really well - I folded the flap around to the back of the phone when making calls with no problems.
After reading on Apple Support that the REAR microphone might be the issue here I called my wife and moved the cover flap during the call. My wife tells me that it makes a huge difference. When the flap covers the REAR microphone calls are muffled or "distant" - when the REAR microphone is uncovered calls are crystal clear.
I was assuming that when on a phone call only the bottom microphone was important since its the one directly in front of my mouth. However, the above indicates that it is just as important to make sure the REAR microphone is also uncovered when on a call.