Seeing a post just now about POTS line current reminded me of back in the 80s when a large customer (Geest) who had an IBM 3750 private telephone exchange system, capable of 100 lines and 1000 extensions I believe. Pye had installed a tele-radio connect system to the PC1 controller so that internal phones could manually be connected to mobiles. The manual bit was due to the fussy licensing of pmr systems back then. This was not usually a problem as the company exchange had one supervisor and at least 2 operators at the console throughout the day. However, as time went on, sometimes busy periods or tea breaks meant that although the "phoned in" request for a connection was dealt with promptly, the "disconnect" at the end of the call (which was manual, a press release of a button on the controller a few feet away) was sometimes forgotten. This resulted in the clear-down tone being transmitted over the air for sometimes several minutes.
As a field engineer and this was one of my big 5 customers locally, I came to the rescue. I unofficially modded the PC1 controller. The connect switch button was modified from latching to momentary action by removing a spring I think. This was then used to fire a thyristor when momentarily pressed so as to hold a relay (in place of the switch contacts) thus making the connection.
Upon the call terminating, the clear down tone ( around 800Hz
) was detected by a 567 pll tone detector on veroboard. This was used to produce a negative going pulse to unlatch the thyristor.
All the years I attended this customer, it never once gave a problem.
However, had the DTI (in those days) known of this unofficial mod they would have had a fit!
Rob