there won't be a fix, because Apple is not aware of the problem
in the Apple store i was told: quite a few people came reporting this issue already, but were sent away with the "it's the carriers problem" excuse.
so there you have it, Apple doesn't acknowledge the issue and blames the carrier. But the carrier really has done squat, it's the iOS 6.1 which has brought the issue...
but yeah, no awareness, no acknowledgement, no fix
so i insisted on a replacement phone because this could take weeks, months...
Same here..on vodafone india...ever since i upgraded to iOS 6.1, 3G has become unstable...full bars, 3G icon everything there but no connectivity
I guess only 2 ways out
1) wait for Apple to come out with a fix (which may eventually come but may take till the release of the next iphone)
2) restore from old backup in DFU mode