The International Subversives

If he turned the volume up on his modem when his computer dialled what seemed to be a dead or non-existent number, he sometimes heard a soft clicking noise after the disconnection message. The noise sounded like faint heartbeats.

Curious, he experimented with these strange numbers and soon discovered they were disconnected lines which had not yet been reassigned. He wondered how he could use these odd numbers. After reading a document Mendax had found in Britain and uploaded to The Devil's Playground, another BBS, Trax had an idea. The posting provided information about CCITT #5 signalling tones, CCITT being the interna- tional standard – the language spoken by telephone exchanges between countries.

When you make an international phone call from Australia to the US, the call passes from the local telephone exchange to an international gateway exchange within Australia. From there, it travels to an exchange in the US. The CCITT signal- ling tones were the special tones the two international gateway exchanges used to communicate with each other.

Telecom Australia adapted a later version of this standard, called R2, for use on its own domestic exchanges. Telecom called this new standard MFC, or multi-frequency code. When, say, Trax rang Mendax, his exchange asked Mendax's to 'talk' to Mendax's phone by using these tones. Mendax's exchange 'answered', perhaps saying Mendax's phone was busy or disconnected. The Telecom-adapted tones – pairs of audio fre- quencies – did not exist in normal telephone keypads and you couldn't make them simply by punching keys on your house- hold telephone.

Trax wrote a program which allowed his Amstrad computer to generate the special tones and send them down the phone line. In an act many in the underground later considered to be a stroke of genius, he began to map out exactly what each tone did. It was a difficult task, since one tone could mean several different things at each stage of the 'conversation' between two exchanges.