Actually a good question that made me think and I found the solution I believe...

Of course with bluetooth !
A Nokia bluetooth headset maybe....
But my hunch is; just a normal Mobile phone from Nokia that we all use with new software, you dial your friend true the phone book in your Mobile Phone menu and choose "VOIP"

Don't forget the tablet comes without addressbook program and I found it strange that I can't access the sim card directory of phone numbers (apart from the files such as picts).
But your question triggered my answer on that pending issue.

Your phone will stay a phone with all the today's features. (addresses, camera, sms )
Calling somebody from your phone goes faster then with the tablet with one hand and finger.

So ! The tablet is just an accessory of your phone !
After all Nokia is not going to put themselfs out of business...