For companies that are thinking about deploying Mitel 3300 IP Communication Platform, if you have Exchange 2007, then you might want to consider integrating the two. Like some 3CX the 3300 does not require any gateway for Exchange 2007 integration:

Mitel’s embedded SIP integration eliminates the need for a separate SIP gateway as a go-between from a 3300 ICP SIP connection to an Exchange Server 2007 resulting in support for multiple forms of Unified Communications including voice, email and fax. Mitel can deliver this capability to existing 3300 ICP customers through a software upgrade that simply delivers the SIP server’s (gateway) functionality resulting in reduced complexity, time and cost for our customers. Native support of SIP on the 3300 ICP enables customers to take full advantage of the deployment of open standards and maximize their investment either in a Mitel or a multi-vendor environment that supports an open standards approach.
-Mitel.com