Back it the early 70's there was an article in Esquire Mag. about "The secret of the Blue box". It about these "Phone Freaks" who figured out to grab a trunk line and make calls around the world, by using this device called a blue box. It produced tones to trick the phone system into routing calls.
I made one but could never get past grabbing the trunk. each number was a combination of two tones. I had gone down the street to a local Electonics shop and used the guy's O scope but never could solve the problem.