I was driving down the road yesterday (passenger), we were just beyond Palmerston North CBD limits, tried to make a call, at this point the signal was getting very low (1 bar).  Running in dual mode, one would have then thought the phone would then kick over into 2G mode (there's plenty of 2G signal).  But instead it dropped the call, and took about 2 mins to finally roll into 2G mode.

Is there a sort of setting that does this?  Am I better off to force it to run in 2G mode for something like this?

It's a Nokia E51 if it makes any difference