This "bad voice call" problem would NOT be expierenced by the iPhone 5 callers themselves, but by the persons at the other end. If your iPhone 5 calling any other phone, that person should experience your voice fading in and out, muffled, "robotic", "digitized", etc. But you may not hear anything wrong.
If the receiving phone is also an iPhone 5, then situation is the worst. I can see husband and wife getting divorce over a bad phone call. Especially if you have sensitive ears (musicians, singers), then it becomes insanely intorable.
Also, if you do have sensitive ears, the "vertigo" effect of noise cancelling which would produce "counter-noise" sound waves into your ear can drive you crazy. Why? Because noise cancelling only works correctly with 2 ears, but this case, one ear is subject to noise and one ear is not, leading to listeners feeling imbalanced and disoriented and dizzy.
iPhone 5 are not widely sold yet, so you would not have to talk to many other iPhone 5's. But soon, I suspect as more people talking on iPhone 5's, the problem will be more recognized and maybe Apple will do something about it. Also, many young users are using iPhone 5 as a mini computer, they could care less about phone calls - texting, email, Facebook, Twitt, web surfing and games are all they want.
I would suggest users just stop exchange for another iPhone 5 to solve this problem (don't you think after the 5th exchange is enough?) and wait till Apple to fix it.
Or go back to iPhone 4S. That's what I did and it works for me even I know iPhone 5 is a better "computer".
So that proves the point: iPhone 4S does not have this "bad voice call" problem. Apple, are you listening?