In reply to mathorn: I used one on Malbogies in September 1980 - was owned by a guy called Ray Dring (who would be about 80 now if he's still with us) and I think he bought it in that year.
You basically had a battery pack on a lanyard and a microphone/earpiece that velcroed to the inside of your helmet. This plugged into the end of the rope through the middle of which was a tightly coiled cable so that it could "stretch" with the rope in the event of a fall - this was "tested" by Ray.
The rope was orange if I remember correctly and felt pretty much like a normal 11mm.
On Malbogies it was great as you could easily hear each other over the noise of the traffic. Ray had also used it on sea cliffs.