Now I own an iPhone and from what I can tell it does the same thing. The 3G symbol displays, but sometimes by the time the call is finished it just has the small circle besides 'VodaNZ' which I assume to mean is the symbol for 'not 3G' (please correct me if I'm wrong on this).
I live in Takapuna, have a strong 3G signal, and work in the inner-city suburbs which also generally have an excellent 3G signal except for a couple patchy areas which soon disappear as you drive through them. But regardless of whether I'm at home or at work or driving around it's still the same.
I can put the speakerphone on which allows me to watch the phone to see what's happening, and the bars don't drop down to nothing then spring back up again, it's just the symbol that changes.
So is someone able to explain what is going on here?