I purchased a new Iphone SE & on some calls I was told that they couldn’t understand me or I sounded muffled or like I’m under water. I sent that phone back & received a new one. The new phone has the same problem. With both phones I tried turning off the noise cancellation & I put tape over the microphone on the back side – I will get one or two calls that are fine & then I’m told again that they can’t hear me. Sometimes it changes during calls.