One of Exchange Server 2007's most significant new features is Unified Messaging. In short, Exchange 2007 can supplant your organization's existing voicemail system and provide your users with all of the same services... and then some. In order to provide these services, your Exchange 2007 server must integrate with your PBX. Exchange 2007's native unified messaging communication protocols are Session Initiation Protocol (SIP) and Realtime Transport Protocol (RTP). Since not all PBXs are SIP-enabled, there are a couple of ways to go about achieving connectivity between PBXs and Exchange 2007 servers.
IP-enable the PBX
Depending on the PBX, it may be possible to upgrade the device to natively support SIP. In my organization's case, we have a Nortel Meridian 1 switch and a separate voicemail system that has reached end-of-life and it no longer supported. This summer, we are upgrading the switch to Nortel's software version 5, which will SIP-enable (VoIP-enable) the PBX and allow it to communicate directly with our Exchange organization. Our Exchange organization is currently running only Exchange 2003, but will also be upgraded.
Install an IP/VoIP gateway
In many cases, it's simply not feasible to upgrade a PBX. PBX upgrades can be costly and complex and not all units can be upgraded to support SIP/VoIP. As such, Exchange 2007 also supports gateway devices that handle the translation between many PBX models and SIP. One such vendor, Dialogic, sells a wide variety of devices designed to support many different PBXs.
For more information on a gateway that might work in your environment, visit Microsoft's Telephony Advisor page.