What phone functionality isn't complete again? He's asking if he can answer calls while doing other things. Yes, you can. I've never missed a call or a text, which is more than I can say for several other consumer level phones I've used.

I missed that a bit too, but there is an app in devel that lets you do that now. Hooks into the existing ring system and plays a custom ring tone when you get a call based on caller ID. It's not as integrated as one would like (eg in the contact card) but it is possible.

To the original poster:
If you're looking for an awesome PDA/computer that can do just about anything you want, that can also get/receive calls and texts and use GSM/3G internet, this is your device. If you want a phone with lots of "phone features" like individual ring tones, call monitoring, local voicemail fast forward sliders, etc, then this is not the device for you. Functionality wise, it can do what it the promotional material says in can do, just don't assume it can do things not listed. There are apps to fill some of the gaps (MMS, alternate IM protocols, etc) but if there's something not listed that you absolutely need, investigate it here first. It seems you're doing that, which is good. Hope you find the right device for you, be it a N900 or not.