I assume others are aware of the "back-dialling" that could be performed with the Push button A / Push button B telephone boxes? 
Aware, yes, but I was more of an inveterate 'tapper', with quite a high success rate. It helped that the dial could be used for '9' and '0'.
Another favourite wheeze was dialling into the GSC and back out, then dialling '0' for the operator. This meant that the call would show on a white light (ordinary line) instead of a red light (coinbox). The requested call would be placed, and ticketed to the coinbox number.
In the days before the 'cuckoo' beeps were introduced, I'd sometimes call my mate at a coinbox and reverse the charge.
But for me and many others, phreaking was about phun, not prophit. Like exchange hopping for eighty miles across country, just because one could - a conversation wasn't possible. Then when the called party cleared, there was the sound of selectors dropping out one after the other, getting louder each time.
Then for a bit of entertainment at lunch time, I'd dial into the GSC, back out, in again, back out, etc etc, just to hear the clicking of the relay sets, and the cascade of selectors when released. Of course, if the chain didn't complete, that would provide an investigation for later in the day.
Crikey ...... SALT (Subs Apparatus Line & Test), wonder if this is still active ?
Not that I'm aware of - 17070 seems to cover everything on the various options, and more.
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Topic: Running two BT phone lines over a single extension cable (Read 17025 times)