This article from TechRepublic looks at upcoming Microsoft VoIP products:

With its new Office Communications Server (OCS), the company is
setting out to get its feet wet in the VoIP space — in a big way.

OCS is about more than VoIP, but VoIP is a big part of OCS. Its VoIP
capability is what sets it apart from its predecessor, Live
Communications Server (LCS). You can integrate the software-based VoIP
feature in OCS into existing PBX systems, or you can use it alone.
Here’s a closer look at OCS, what it does, and what it means to
companies looking to deploy VoIP in their organizations.

Two editions for two different markets

OCS, like many other Microsoft server products, comes in two
different editions to serve different markets. Small and midsize
companies can get into the game at a lower cost with the Standard
Edition. Not only does the software itself cost less, but it runs all
the components, including the database that stores user information, on
a single server computer so hardware costs can be less as well.