Microsoft has released its Office Communicator Mobile application which offers secure instant messaging and VoIP capabilities to mobile devices. Communicator Mobile offers a similar look and feel to the desktop version of Microsoft Office Communicator 2005, and it ensures that access to capabilities that are provided by Live Communications Server 2005 continues when users are away from their desks.
It is available in four different versions, depending on which operating system your device uses (Windows Mobile 2003 SE for Pocket PCs, Windows Mobile 2003 SE for Smartphone, Windows Mobile 5.0 for Pocket PCs and Windows Mobile 5.0 for Smartphone). Note that VoIP is only offered on Windows Mobile 5.0 Pocket PC. To make use of this application, you also need Microsoft Office Live Communications Server with SP1.
Source: Pocket PC Thoughts