the idea sounds alot like SELCAL used in aircraft who use HF. Since they use SSB radios, they will use a DTMF tone to break the squelch as well as alert them that a call is coming in for them.
I played around with adding SELCAL to two old AM Radioshack CB's a few years ago. It worked pretty good. All it did was just unmute the speaker when the proper tone was received. Many DTMF boards can be wired for this
or this TAPR kit woudl do what you need
http://www.tapr.org/kits_das.html
I do remember reading about an exension speaker with this function built in , but forget who sold it. i think it went for about 50 or so.