I think my wife and I have figured this out. If it has already been mentioned, sorry. I didn't read the whole thread but wanted to let you know what we discovered. As you are aware the iPhone 5 has 3 mics. One on the bottom you talk into, one in the speaker grille where your ear goes, and one on the back near the camera. They all help in noise cancellation. The one by the camera I tend to cup my hand around when I talk using my left hand but not the way I hold it when I use my right hand. I think most right handers hold it the same way when using their left hand. If the speaker hole by the camera is covered completely it doesn't seem to be a problem but if I cup my hand around it causes an echo-cave-seashell effect on the mic and it complicates the noise canceling which results in your speech sounding muffled to the other person. Try it. Call someone who will help. Keep the back side mic completely clear or completely occluded and they have no complaints. Put your hand over it and they say you sound distant or muffled. Anyway, that's what we found. Remember "it's not the design of the phone, it's how you hold it" hahaha! Memories. Still love the phone and now that I realize that this mic must be completely open to the surroundings I have had no issues from anyone I talk to.