Re: Calling OfCom and Openreach...
100% correct re "built-in from day one comment". The PRX-205 exchanges installed in Saudi Arabia in the early 80's all had line monitoring built in. Each exchange could monitor 4 lines remotely from Riyadh in a secret underground bunker between Airport and Pepsi roads.
In the UK there used to be a small room in all exchanges with operators/switchboards with a 'Miss Moneypenny' type lady who could be guaranteed to be discrete. Not that it mattered because the GPO used to put interrupt tone on the line....
Not mentioning Menwith Hill and mobile phone monitoring, that is a different thread.