In my case, i must hold phone straight to the mouth then people who talking with say the call quality is ok. When I tilt the phone away from my mouth just a little bit, then everybody tell me they feel like I'm speaking through the material or or was in a swimming pool and they hardly hear me. In my opinion this is software problem, cause people from asus forum says when use whatsapp or messenger call quality is ok. I didn't test this yet but probably do this. I think the solution will be to find good alternative app for gsm calling.