I-phone 6S Plus

If you have distorted Sound issue on your iPhone --> If you hear no sound or distorted sound from your iPhone, iPad

If you are not able to hear the other person, click this...>If you can't hear a person on a call or voicemail or if the sound isn't ...

If you only hear through speaker phone mode --> If your iPhone is stuck in headphone mode - Apple Support

If the other person is not able to listen to you then please click this--> If the microphones on your iPhone, iPad, and iPod touch aren't ...

If the above options do not mitigate the issues then Contact your carrier and then Contact Apple Support

A large number of users are reporting this problem on different iPhone models. I believe I have tried all of the suggestions that have been made in the responses to these low volume reports. I have to use the speaker to hear most phone conversations. I have reported this issue on the Apple feedback page. It would be helpful if you reported the problem there as well. That's the best way to report this issue. Apple does not monitor this forum.

iPhone 8 / iOS v13.6