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.