Make me really want to build one for my son when he gets to that age.

If I build it, I can control the full feature set and explain to him how it worked and he'll get the 'cool' factor too. With the raspberry pi I have lying around at home, it doesn't sound impossible!

It is really straightforward with some cheap hardware and patience. I have another comment on this thread with more breadcrumbs but I used a Fanvil "hotel phone" and voip.ms. Under $50 and an afternoon all in and you have full control.