Volume

If you have a 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 speakerphone 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