As I recall, the first to exploit this was Joybubbles; however, Tuftes may have been the first to build a blue box. The information on MF signalling was freely

As I recall, the first to exploit this was Joybubbles; however, Tuftes may have been the first to build a blue box. The information on MF signalling was freely available in 1960 when it was published in the Bell Tech Journal so it is conceivable that there may have been others who developed this in parallel. For those who are interested in more on Joybubbles, then there's a documentary being made about him. More info at http://www.joybubblesthemovie.com/ http://www.joybubblesthemovie.com/