I have Internet Call setup with my Vbuzzer account on my N800. (2008 OS) It works very well. I must say the sound quality is better than I expected even without headphones.

I recieve incoming calls only if the N800 is active. If it is in sleep mode ( I have not touch it for a few minutes, ie screen is off and I am still within range of the connected wifi AP) the calls just go through to my voice mail. Is the a setting to wake on incomming call?

Also how do you get a dial pad during a call? often you must navigate menus where you press 1 for this or 2 for that. I can't seem to open the dial pad during a call.

I use a SIP account as well and the quality is as good or better than a cell phone. However my N800 will wake up when I get a call. I often have the screen locked and in my pocket, the ear bud mic/switch allows me to take the call without touching the device. I've often had an inbound call while the device was sleeping. Are you using the builtin client that shows green on the presence indicator? You might need to visit some advanced options, I didn't need to for mine.

The dial pad, that needs to be fixed to be more obvious however you'll find it under the top left menu after a call is started. Under Internet Call / DTMF dialing. That pulls up a dialing pad that at least on my system generates the proper DTMF codes you'll need.