I have seen the same flaky 3G dropped call/call failed problem in my office. When I had my BB 8310 I loved it because I had a dropped call about 3 times a year, seriously. It was bulletproof. The first week with my 3GS I had 5 dropped calls. Not a good user experience. It almost went back to the store. I now have learned that while in my office, I set it back to EDGE, and I get decent connections, can make and receive calls, and get much better battery life as well!
See, all I had to do was learn to serve the iPhone and the ATT network, rather than make them serve me. Silly me, I thought I was the customer, and paying the bills......