I served in the Auxiliary Fire Service at Kings Norton Fire Station for a few years and remember going up the Lickey Hills one summer's evening to try out radio relaying. I'm going back to the 1960's so the memory's fading a bit but to prove the system, we used a combination of three vehicles - a controller, a relay station and a receiver. Things were quite basic in those days and nothing as easy as today's phone systems which seem to be able to reach anywhere!
In fact, for convoy work, (another of our specialities, and one which I'll elaborate on) we still used dispatch riders on motor bikes!
I can also recall a training trip we made to Edgbaston Reservoir to try out our "Bikini Unit". Another story I'll relate if asked to do so.