Most of xiaomi phones have mic problem especially while calling the other party is unable to hear properly. I'm experiencing this in my redmi note 4 but similar issues were faced on some of the Xiaomi devices in the past as well and it's believed that it's a software issue.
I'm going to talk about the redmi note 4 mic problem solution which I found on mi forum and tried myself and wanted to share it with you . It's applicable to any xiaomi device like redmi note 3 ( The predecessor had the same mic issue too by a huge margin) or basically any xiaomi device. I hope you like it.