I don't doubt what Peter Hennessey found in the Archives but I wonder if it was founded on ignorance and secrecy. On the Ops Desk at Waddington was a black telephone. It had no dial, and as far as we knew, no purpose. It never rang; we never used it. The number was Lincoln Sub Trunk 2.
We decided to try it.
One day I rang the operator, asked for the number, moments later it was picked up on the Ops Desk. It was obviously a super efficient network but one of which we mere cannon fodder were ignorant.
Maybe that was what i t was intended for.