Make something that looks like whatever - possibly like a cellphone even - only make it wireless buy using (maybe bluetooth?) built into the fake phone and probably as a connector that plugs into the handsfree socket on a real cellphone. Or better yet, maybe just use a bluetooth cellphone to interface with.
Then you could make the 'fake' cellphone really interesting. Set it up with a little rotary dial, maybe. With 12 positions instead of 10, of course, to handle * and #. Maybe set up some small numitrons or nixies as a display. Possibly some big shiny toggle switches for the send/end buttons. Perhaps make the casing from bakelite or something similar. All sorts of neat things you could do.