I have also wondered (but do not have the knowledge to poke around) whether some software exists to connect and use the built in Voip app on the N8x0s as a handsfree/dialler console connecting to a phone over bluetooth. I have used VOIP-VOIP over bluetooth and 3G, and it is reliable and robust.

We almost need something like an asterix server on the phone for the N8x0s VOIP client to connect to and route the calls out as a regular call, and when receiving calls to forward the calls onto the voip client on the tablet.
Heaven only knows if anything exists though.