IPhone 4s sound quality for caller?

When I call people I can hear them clearly but my voice sounds very distant or muffled to them, however if I put them on speaker they can her me clearly. Assuming the same mic is used at all times (AFAIK there's only the one mic on the iPhone?) then it's not a hardware problem. So what's the issue, is it something I can correct in the settings?

Http://www.idownloadblog.com/2011/11/01/iphone-4s-audio-echo-during-call/
http://www.idownloadblog.com/2011/10/16/iphone-4-vs-iphone-4s-speaker-volume-video/

Went to Apple and they replaced my phone - used the same SIM card. It seems to be working well now, with some minor voice glitches here and there. I just chalk that up to the cellular network I guess.

wkr p

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