There were some UM questions posted to the Ask a Question section of Exchange Ninjas 2007 Wiki a couple of weeks ago.  I thought that this information might be helpful.

Q: What PBX’s gateways does Exchange 2007 currently support?

A: Initial support will be for Intel PBX-IP media gateway (PIMG) and AudioCodes Mediant 1000 and 2000 IP/VoIP media gateways.  These two vendors support a large varitey of PBX models and configurations.  Others vendors such as Cisco Call Manager may function however specific guidance and support for these platforms has not been announced by either Microsoft or Cisco.

Q: Does Exchange 2007 require CSTA compatibility with the PBX gateway? What PBX’s gateways support the WMA and Exchange 2007 format?  Is that a CSTA standard?

A: The gateways utilize SIP to communicate to the Exchange UM role.  The interface from the PBX to the gateway can be a T-1, E-1, station emualtion or even analog w/SMDI

Q: How does a PBX forward WMA files to the exchange server?  Through the gateway?  What format is used?  Where is this configured in Exchange 2007?  Where do we configure this on the gateway?

A:  The gateway establishes a SIP/RTP connection to the Exchange UM server.  The server handles the conversion of the SIP/RTP conversation into the proper file format for storage in the mailbox.

Q: Does ISA 2006 do that gateway feature or are we stuck buying 3rd party devices?

A: ISA Server is not directly involved in any Unified Messaging function.  Currently to be supported a third party device will need to purchased and configured to use the Unified Messaging features.  ISA can be used to publish the UM web service which is used by Outlook 2007 to "Play-on-phone" and configure UM settings.