Now this is useful - I get loads and loads of questions about this subject. If you have a Dialogic gateway and are thinking of Exchange 2007. Dialogic has rolled out it's new website here. This website focuses on Dialogic Gateways that are certified for integration with Exchange 2007 Unified Messaging. These gateways support popular integration options to telephone switches too.
Eight to 120 ports of Voice over IP and Fax over IP connectivity per gateway
Tight integration to leading legacy PBX's from Nortel, Siemens, Avaya, Alcatel, Mitel, Ericsson, Siemens, and NEC
Easy installation and maintenance with web-based configuration and remote management capabilities
Investment protection for legacy equipment that also enables worker productivity enhancement solutions such as Unified Messaging
Support for SIP and SIP supplementary services such as message waiting indicators, transfer, hold, park, and caller ID functionality