The "ear and mouth" part of the phone interface can be a thin strip which you hold with your hand. Picture something like a short drinking straw, curved in a good way.

With a little design, that could attach to the side of the tablet (snap in, so it doesn't get lost - could also charge itself if it needs much juice).

So you want to make a call, just snap off the 'handset' and talk. All the social cues about phone calls apply.

It might also be possible to engineer it as a stylus, but there may be a straight/curved issue to resolve there.