What you describe is not just a problem with the N900, this is absolutely an issue for all modern mobile computing platforms (aka mobile phones)

The telephony aspects of the N900, and others, are handled by separate processors. In simple terms this means that Maemo can crash out completely, but in the depths of your device the IC's responsible for network connectivity are still functioning just fine. Obviously maemo has died so it's simply not able to notify you of the incoming call.

This does not mean that your device can't acknowledge the call on the network - it explains why your (forgive me) ignorant friend rants about 'smart' phones not being so smart after all. The same thing does happen with <insert any brand you like here>

If you want a modern device to notify you of calls no matter what, then you'll need to go a bit retro - Nokia Series 40 or older : )