Well you probably couldn't answer the N900 while on the laptop unless you had some SSH or VNC connection setup to the SSH to remote control it. But what PulseAudio's networking lets you do is pipe the audio to whereever you want.

E.g. if you wanted to play whatever sound that was coming from the N900 (music, podcast, and ideally a phone call) you could direct it over to your desktop's speakers or laptop's speakers). Not sure how it works with input sources but I believe it could be the same (if it is you could use say your headset or laptop's microphone instead of the N900's microphone).