Can your phone system talk to Exchange 2007?
It's been online for a little while - I only really noticed recently, but there is a matrix of telecom PBX systems and VoIP gateways which can sit in front of them, in order to enable Exchange 2007 Unified Messaging.
The Telephony Advisor for Exchange 2007 goes into some detail around what needs to be understood in order to get UM running. There's a link on there to the PBX Configuration Notes page, which not only details how the PBX/gateway needs to be configured, but shows a list (ordered by PBX) of what components and protocols are used, which versions of software are required etc.
It's not an exhaustive list but is a starter - if you want to know whether your current phone system could be integrated into Exchange 2007, hae a look...