iphone XR microphone muffled during calls

I just realised that I have this problem recently (when I was on iOS 14.5). Now I'm on iOS 14.5.1 and the problem is still there. There can be a possibility that this problem has been around for longer but just no one told me earlier.

When I'm on normal (cellular) phone calls, those on the other side of the line tell me that my voice was muffled and they can barely hear me. When I switch to use the speaker phone or hand-free, my voice becomes clear. I also tried using Whatsapp/Signal calls and my voice is all ok.

Searching the web, I found lots of other users with old iPhone models (e.g. iPhone 7) had exactly the same problem. Some of them suggested sticking a tape on the rear-facing microphone during a call. I followed suit and really my voice becomes clear on the other side again.

I've already tried turning OFF the noise-cancelling function under the cellular setting but that made no difference.

While putting tape on the rear mic seems to solve the problem, however, every time I need to take a video with the rear camera, I have to peel off the tape or else the recorded sound in the video will be unclear.

I'm so tired of this endless sticking and peeling when using my phone is under normal daily use.

Would appreciate if someone out there can help!