You know traffic lights are controlled over analogue private circuits, don't you Terry?

So Terry is doing some work in an exchange.

He's trying to locate a pair he needs to connect a customer.

He shorts out one of the pairs with his pliers, and hears a screeching sound
outside.

"That's weird" he thinks. "Wonder what it was". He thinks some more.
"maybe if I try it again the same thing will happen".

He shorts out the pair.

Sure enough. "SCREECH!!!"

"Wow!", says Terry to himself. "what's this????"

Terry knows his mate Fred is in the exchange, and is an old hand at this
sort of thing.

So Terry goes to get Fred.

"Fred, take a look at this". If I short out this pair here there's a weird
screeching noise comes from outside the exchange!"

"What", says Fred. "Show me".

"Sure", says Terry and shorts the pair out.

"SCREEEEECH! CRUNCH!!!!!!"

"Blimey", says Fred. That's sounded like it came from just outside. Let's
take a look."

Fred and Terry swipe themselves out of the building (separately of course -
otherwise they'd never get back in).

The sight that meets their eyes is one of disaster. Two cars have collided
at the crossroads outside the exchange.

"Oops!", says Fred. "You know traffic lights are controlled over analogue
private circuits, don't you Terry?"