For people wondering what UM is, its basically the ability for everything to be in your Outlook inbox - email, fax, voicemail etc. And is a server role in Microsoft Exchange 2007. You can read more here. Please note this post if not for the feint of heart as Exchange is a complicated beast, this is also not for a thread to flame MS.
From what I understand UM connects to either an IP Gateway, or an IP PBX and provides a connection to store voicemail etc in the Exchange Store file rather than the traditional way. Microsoft has a list of recommended / tested IP PBX and gateway systems - alot of which are made by Cisco, and since Linksys seems to be the VFX manufacturer of choice, they may be compatible (Linksys being a division of Cisco).
We have signed up for Xnet and VFX yesterday and I asked the same question of them on the phone, but haven't heard back from then yet. I thought posting this on the forums might be a good way of finding out if any IT Pros in NZ have done this using VFX.
Look forwarding to hearing your responses.