O2 3G on my 7600?

[Adam]

Hey, got myself one of the cheap 7600's on 3, unlocked it and been using my O2 SIM card in it.
I live in Sheffield, and 3G coverage here is very patchy on Vodafone and O2. If you keep doing a manual search occationally it will show the '3G' bit next to the operator name on the 'select operator' screen.
But anyway, was on the bus today with my phone in my hand skipping through the MP3s when my 2G signal went to nothing, and no operator name was on the screen. But then O2 poped up again and under the signal bars(of which there were only 2/1) it had the 3G symbol. I tried to call but it kept on saying that the call was barred, then cut me off back to the 2G network.
Why did it log onto the 3G network?

[graale00]

Well you were probably in an area where O2 were testing there 3G and seen that your IMEI was not registered to use 3G so didn't allow your call.

[faf1]

You will need to set your 7600 to GSM only to avoid this, it will look for a 3g signal in preference of 2g. Your signal will drop off as it attempts to log onto the 3g network, when it fails it will look again for a 2g signal.

It will be the SIM which is not registered for 3g services. Only SIMs which allow voice calls at the moment are the engineer test phones.