I'm still having this issue, and it's driving me nuts. I'm on 4.1, and called at&T. They won't tech the issue further until I update to 4.2, as they said there is a carrier settings update that was added. There is no reason for me to update, and I don't want to with being jailbroken. There has to be countless people that are still even on 4.0 and don't have a dropped call issue.
Any other ideas?