You are comparing mechanics with electronics.
They don't optimize the same.
You only have to add a microphone (better make it stereo for good
quality) and a speaker.
Oh now you have a perfect voice recorder! You can use that quite
handily for keeping notes.
Perhaps if we add wifi we can use it to accomodate incoming calls.
But of course it would be nice to call them back if you missed them.
Perhaps a touch screen lcd so the input will not be so arduous?
Don't forget the answering machine.
Of course since we can playback video and it has wifi - why not give
it internet access?
A MP3 player is a given - just need a little more firmware - of
course it can play movies - who wouldn't want that?
Actually you might as well add in a calculator and email - because
what simple piece of electronics doesn't have that?
You know what - a camera is like a scanner. And since we already
have the emails - well it would make a dandy fax machine. Hmmmm.
How do we receive faxes? Well it would not take much real estate to
put in a dandy little printer.
Sorry darling I can't take pictures of the wedding right now - the
priest says to turn off all cell phone type devices...
.... tongue in cheek of course - no harm meant .....