Yes, dating back to the earliest phone systems it was necessary for the operators to listen to a phone line to see if it was busy before connecting (especially

Yes, dating back to the earliest phone systems it was necessary for the operators to listen to a phone line to see if it was busy before connecting (especially in an emergency). And this carried through to the step-by-step switching systems where there was an extra level in each selector bank to allow the technicians to check the status of a line before working on it. It was only necessary for the tech on the test desk to dial a number to be able to monitor it. And of course this monitoring system was "unofficially" connected country wide to the spooks office. And with modern computerised exchanges, this facility is just a few extra lines of code in the software.