As the current documentation for the Office Communicator Mobile 2007 R2 client for Java (used by Nokia S40 and S60 devices, among others) is a bit sparse in regards to client setup, I thought I would share this with you.
As you are probably aware, CoMo R2 for Java utilizes components from the Communicator Web Access server, but the thing you need to be aware of is this:
The Server Address (URL) that you enter when configuring the client on your mobile device has to point to the Communicator Web Access URL, not the Access Edge (typically sip.domain...)
Thus, if you need to support Java clients using CoMo R2, you need your CWA published externally through a reverse proxy (ISA/TMG recommended) - and those clients need to input a different URL than the Windows Mobile CoMo R2 clients (which use the Access Edge URL).
Hopefully, this will be spelled out in more detail in the documentation in a future revision.