Seriously, no one's posted the Lego version of this already? Actually looks like the Lego bot does better depictions to the (un) trained eye ... http://robotics.benedettelli.com/portrayer.htm

I'm sure someone else will correct the details, the 5v on the line will power normal phones to work as an intercom, getting them to ring is a bit trickier. I think it's historically a 80V pulse? Since I don't see any futuristic good ideas, go retro. Get a couple of old hand crank phones, (there are old desktop models reasonably priced) disconnect the line at the incoming box, add a 5v wall wart to power the line. Then when you want the kids to come to dinner, you crank the phone and they'll pick it up because it's novel.. works for a few weeks at least! Actually inspired by a friend who had wired one out to the small guest/kids house, ours is just upstairs/downstairs.