How to control which network is used for IP data in WM6?

I noticed - simply by operating my new WM6 mobile phone (HTC TyTN II)
close to my desktop speakers - that when I am using the built-in Pocket
Internet Explorer the device constantly chatters with the GSM network
even if I am connected to my local WiFi network. Judging from the
accoustic pattern it doesn't seem to send the bulk of the data over the
phone network but there are frequent short exchanges, e.g. each time I
click a link in PIE there is some short "di-di-di di-di di-dii-di"
from the speakers, as if, e.g., the address resolution is done via the
phone network and the actual payload then downloaded via the WiFi
network.

How can I control that ALL data is REALLY routed via my free WiFi
channel and not via the (alas, in Switzerland prohibitivly expensive)
GSM/UMTS carrier network? I am fearing that I will end up with an
enormous phone bill end of the month if I don't make sure that I am not
unvoluntarily using the phone connection all the time.

I know that I of course I could disable the phone functionality and just
enable WiFi while using the browser or checking my email, but that is
not the point here. I want of course remain reachable for actual phone
calls while using the device to look up information or reading emails. I
just want to be able to control over which channel my *data* connections
are routed.

Michael

Just noticed, that I had out this append into the wrong NG. It has
nothing to do with ActiveSync.
Moving over to PocketPC NG...

Sorry for the bandwidth!
Michael