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]
I too have had multiple problems after the update with calls failing immediately after dialing, many dropped calls, and also not hearing anyone on the line while calling. It was noticeable before the update, not it is very much amplified. I was just thinking, as an internet/multimedia device the iPhone is awesome. As a fun, not so much right now.