High pitched sound when in voice calls (Not during Speaker mode)

Only during phone calls (making or receiving), there is a distinct high pitched noise. Which is really uncomfortable to bear for the receiver, caller.

I’ve tested taking videos (nothing wrong there), Making voice memo (nothing wrong there). Even playing music.

However, if I select the Speaker mode it goes away. As well as if using an earphone with mic plugged in.

Little history on the device:

The phone front screen was finally shattered (after many years), and took it in for repair/replacement (at a local repair shop). It was repaired, and all was well (albeit having to return for faulty screen). This repair/replacement was done last week.

I’ve also installed a replacement battery ordered from this website with success, a day after the repair was done. All checks were good. There were no more issues (besides the minute lack of response/feedback feel from the replaced screen unit by the shop versus the original screen that was broken).

Today, this problem started out of the blue.

I’ve tried to hard reset the phone thinking it was the culprit. The power button + home button, and the DFU reset. Failing that, I have backed up the phone and fully reset the phone as it was out of the box, and updated the phone with back up. That didn’t work either.