This was something that worked on pay phones (in the UK at least) to get free calls. I think it was around the 70s/80s? I can't remember the exact mechanism, bu

This was something that worked on pay phones (in the UK at least) to get free calls. I think it was around the 70s/80s? I can't remember the exact mechanism, but something like the coin unlocked the dial, so if you didn't need the dial you could make the call without a coin.