If you've not already downloaded iOS 6.1 for your iPhone 4s, please hold off for the next version while Apple fixes 3G performance issues. Thanks
The carrier has yet to offer additional information on the issue, but Apple's iOS 6.1.1 update has just entered developer testing earlier this week. Vodafone's message suggests that this update will address the 3G problem, but neither Apple nor Vodafone has explicitly confirmed that this is the case.
Update: Vodafone has now posted a notice on its support forums addressing the issue.
Some customers may occasionally experience difficulty in connecting to the network to make or receive calls or texts or to connect to the Internet. Apple is working on a solution to their software issue. These connection problems are intermittent.(Thanks, Martin!)
While Apple’s investigations continue, we would recommend that anyone who has not yet installed iOS 6.1 on their iPhone 4s should delay doing so until Apple has confirmed that their problem has been fixed.