I haven't tried it w/ the N900. But Broadvoice is touting a new Android SIP application. Since N900 has it built right in, you can probably use these directions:
http://www.broadvoice.com/airdroid-f....html#existing

The jist is that you need to turn on multi-presence and then fill in the right information.

I will give this a shot in the next couple of weeks. So I may start a new thread when I do w/ what I find.