They can’t hear me!

Sometimes when I make or receive a call whoever is on the other line can’t hear me. They say it sounds as if I’m under water. It doesn’t matter if they are on a mobile phone or landline. I traded in my iPhone 7 for a 11 thinking it was the phone, but it is doing the same thing. I don’t know what to do, I’ve called customer support and we tried resetting and other things and it still is horrible!