Network doesnt switch on 3g once it locks on GPRS

Hi,

I'm Italian and I have an iphone 4s with Vodafone as mobile operator.

There is a problem with the network lock: if i am in a room or a tunnel with low signal and the phone switches from 3G on GPRS connection, when i after came in an open field with full 3G network coverage it doesn't switch back on 3G: the only thing you can do to make it use the 3G is to go on "airplane mode" (or switch off the data usage) and then go back on normal mode.....

This affects my friend and me, on iphone 4s and both on IOS6 and IO7..

there is a solution for this?!

thanx a lot

Matteo