I’ll just say it: Lync _is_ a PBX Replacement. And then some.

At the beginning of the year, PointBridge moved to a new office in Chicago. As part of the move, we moved all of our collaboration/communication to Lync 2010. We used to have an OCS R2 <–> Cisco CallManager integration for voice needs. But now – it’s 100% Lync for all employees. And I’m happy to report that it has been a huge success. So to all that wondered if Lync really is a PBX replacement, it is.
Well, actually, it’s not totally fair to say that Lync is a PBX replacement; our PBX was only doing one thing (dial tone) whereas Lync is doing:

• Voice
• Video
• Audio conferencing
• Web Conferencing
• IM / Presence
• Desktop Sharing
• Softphone
• Mini-Contact Center (for our customer support) with call queuing

A comparison of our Lync deployment to most other vendors’ deployment would involve the following chart:

We now have just two Lync servers – Front-End and Edge – and a Dialogic survivable voice gateway making up the entire deployment. You should note that every service runs on one Lync Server by default. It’s not a bunch of different applications you install on various different machines. Lync 2010 is one application that handles all those services. The Edge server is used to make Lync 100% accessible externally with no VPN required; it securely proxies traffic from outside the firewall to the Lync server.
Stick the Exchange UM server in for voicemail and it’s a pretty tidy solution that scales support several hundred users. Plus the Dialogic gateway acts as a high-availability/ redundant Lync service: if the MAN link between our office and the colo facility were to go down, all phones and Lync clients at the new office would stay up, registered to the gateway.

Along with the new Polycom Lync phones (and the usual collection of various headsets) we have an enterprise-class communications system.
If you are looking to scale it out for thousands of users, or want additional high-availability, your design would include more than what I have depicted above. And I realize that not everyone would have need for all the features I’ve described, or maybe have need for features above and beyond what I’ve listed. And there may be even better ways to get the same functionality. But for our deployment, the above is what would have been required for us to meet all our needs.
So I’ll ask an open question: what do you need to do with your PBX / existing vendors to get all the benefits that we are getting with Lync?

If you answer is that you are integrating multiple applications, using multiple clients, multiple servers, and multiple logins/passwords, you may want to consider what Lync brings to the table. A unified infrastructure on the back-end and a single unified client for users means that Lync is a solution worthy of the name “Unified Communications”.