Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
iOS 6.1 Causes 3G Issues For iPhone 4s Users On Vodafone UK
Some Vodafone UK customers who are using the iPhone 4s have been getting messages for Vodafone them selfs warning them not to upgrade to iOS 6.1 due to some users experiencing poor 3G performance
Vodafone UK also posted a notice on their support forums saying
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.
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.