No problems here. All morning I was in a place that kept my iPhone switching between one bar of 3G/full bars of EDGE, and I was talking for a long time with no problems.
I've actually had one dropped call since I got the phone 3.5 weeks ago, which is actually better than I expected when I switched to AT&T.
I've actually had one dropped call since I got the phone 3.5 weeks ago, which is actually better than I expected when I switched to AT&T.