The biggest factor is the area you are in. If one company has better service then the other then obviously the call quality will be better. However, I do tend to agree with you in that in areas with equally good service, Sprint does sound better. I had Sprint for 3 years before switching to AT&T for the iPhone last year. I am definitely not unhappy with the call quality of AT&T here and I have never heard any complaints from other people being able to hear me, but I can tell a difference when talking to someone on an iPhone vs Sprint. I don't think it's a huge deal for everyone, but for some people it may be.