I have iPhone X with Black Apple Folio case. I recently have got numerous complaints that my calls sound muffled and quality is not good. So I called Apple and they ran some test and no hardware issues with phone. Every time I take Folio case off, quality sounds perfect. I even tested making videos of testing voice quality and have found everytime folio case is folder back the mic gets distorted. I believe the magnet built in case is causing interference with mic. Going to Apple later today to show them exactly how case effects microphone. Wondering if anyone else is having this issue. If you have Folio case just test making video of you talking with case folded back and normal and it’s clear audio difference.