How to fix crackling/static noise during calls?

My iphone 6s microphone makes static noise/crackling sound/and is kind of muffled during calls and in voice memos too! I’ve already replaced lightning connector/microphone/audio jack assembly 2 times thinking the problem was in microphones. But strangely enough it didn’t fix the problem. When I shake or softly bang the phone against a soft surface or my palm a few times the voice gets somewhat clearer. Do you know the exact solution to this? Is it better to get rid of this phone or try fixing it again?