Not a cellphone more like a CB radio push to talk thing that is hooked up to main stereo.
You and your buddies are cruising down Route 66, all of a sudden the driver notices a really cool looking fruit stand that he can pull of to but is unsure if the rest of the group would like to do so.
The guys in the back are listening to the radio. The driver tries to yell and get someones attention but is unable to over the music and the noise of the bus.
Then he remembers "that thing" in the stereo setup that allows him to talk into the mic (CB radio push to talk thing) and cut out the music while he talks so everyone can hear him. And everyone enjoys good fruit. The End.
Similar setup to what pilots/flight attendants use to communicate with the passengers or like the tour guide uses in the tour buses.
that help at all?