Microphone issue op12

I faced wired issue during phone call..my voice heard very very low not understandabel to otherside of call when I recieve or make call inside car...while outside car all working ok.. evenwith bluetooth device inside car working all ok.it creat issue only inside car without bluetooth..though of mic problem but why inside car only mic problem.. no network problem as bluetooth removes the issue of inside car. I have bought second hand device with immediate after purchase apply tempered uv glass..so can't comment condition before uv glass...hav no bill box purchased at 33k..16/512...1.5 year used ..99% mint condition ..92% battery health.3 month shop warranty for repair.. Affraid of repair as once open it will damage and scratch phone . . Need Genuine suggestions