iDialog provides presence information on both your personal contacts, and across contacts within the corporate address list (which you can search from the client). You can then either launch an OCS Instant Messaging conversation, or use the iPhone to call any of the numbers listed in the contact info.
IM conversations can be multi-party, and the client can support many simultaneous conversations. They are displayed in the familiar threaded text message format from the iPhone.
In addition to IM, the client can make use of VoIP call control to manage voice calls to their OCS VoIP end-point. Incoming VoIP calls to can be forwarded on to either their listed mobile number (presumably the iPhone), voice mail or any other number. It isn’t however a VoIP endpoint in its own right however. Presumably it would not have made it through onto the app store if it had.
From a backend perspective it relies on OCS 2007 or 2007 R2, and make use of the Communicator Web Access server role. Unlike the solution from Web Messenger it doesn’t rely on separate, additional, server infrastructure.
The app is priced at £5.99, which is pricy for both individuals and corporate deployments. However the web site does mention that corporate licensing options are also available that would – I assume – reduce the per seat licensing.