Most of our corporate phones are Telecom phones (Titan, Boss, Touch) which sync to our Exchange server via the Telecom One Office VPN. So when they get into the network they are actually inside our network behind the firewall. The sync works okay.

I needed a GSM phone so got the SE P1i which has an Activesync application clone from Datawiz. Took opening up a port on the firewall so I could tunnel into to see the Exchange server (actually the Outlook Web Access server I think) but then the phone was able to sync mail and calendar fine over the Vodafone network!  The great benefit of this is when I travel around the world, I should still be able to do this whereas for the Telecom phones, the process only works in NZ.