All the calls to my iPhone 4 is tremendously choppy from when I bought it. I am having the AT&T Microcell in my house because AT&T's service on 50' of my street is atrocious. I also found that the calls skip frequently and therefore I figured out that it may b my ISP. I performed a speed test on that and found 20mb down and 12 up. I tried call to some other number and the same thing happens. I changed the sim cards and place it in my 3GS, call working absolutely fine.
Actually, there are two iphone 4 in my house and both have the same thing. Both 3GSs worked without any problem. It cannot be figured out by the AT&T. Think that it may be the antenna or software issue.
If anyone else out here is facing the same issue or you have any solution or suggestion or even the reason for this then please post it over here. I l will be very thankful to all replies.