This is coming from an American, but the 3G coverage should allow simultaneous data and voice.
From Apple's Website:
You can tell iPhone is connected to
the Internet over a cellular data
network if you see , or in the status
bar at the top of the screen.
If you're outside your carrier's
network, you may be able to use a
cellular data network from another
carrier. Roaming charges may apply.
Note: If not on 3G, while iPhone is
actively transferring data over the
cellular data network—downloading a
webpage, for example—you may not be
able to receive calls. Incoming calls
may go to voicemail.
GPRS and EDGE will fail at offering both voice and data, and it might be that you initiated the call on an EDGE network, and moved over to the 3G areas, either way I would recommend resetting network settings on the phone.
Settings>General>Reset>Reset Network Settings
This will reboot your phone, reestablish network connections, and forget your wifi passwords.