This problem seems to be intermittent on my phone and suddenly got more frequent a couple of days ago. Previously people I called never complained of microphone problems but now everyone is telling me I sound far away and muffled.
I became suspicious that the phone microphone is being used instead of the headset microphone, so I did the “walk away from phone while recording” test. I can confirm the phone microphone is being used, not the headset microphone.
I just did the unpair > reset > pair process and I’m still having the issue.