We're not having any problems, but Ars has found many people complaining about the iPhone 2.0.2 update killing their ability to actually make calls over 3G. Among the people who suffer from no-calling-itis is one guy who took his phone to AT&T and had them swap out the SIM, which somehow fixed the problem. If you're getting "call failed" on 3G, try switching back to 2G in Settings -> General -> Network and making the call again. [Ars Technica]
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