It is possible there is an issue with your carrier settings, the provisioning of your account, a network issue in your area, or a defective sim and you should discuss this issue with your carrier.
Since you can record your voice properly and you have followed the tips below it might be worth trying to restore iOS and see if that fixes the problem.
Get help with the microphones on your iPhone, iPad, and iPod touch - Apple Support
If your iPhone, iPad, or iPod touch won't turn on or is frozen - Apple Support
If a restore does not fix it and the carrier has no recommendation, you should make an appointment with the Genius Bar or an authorized service provider to have the phone evaluated.