Same setup here, usually no problem. Once in a while I see those symptoms while I am traversing a border and the iPhone gets in a kind of "undefined state" while switching between networks. Sometimes it recovers itself after a minute or so, sometimes it is enough to disable 3G-reception and sometimes I have to reboot. But all this is always RARE.
AM for some 25.000 km² around Maastricht, the Netherlands
iPhone 3Gs / iOS 6.0.1