Welcome to our comprehensive guide on the fascinating history of phreaking in the Kingdom of Great Britain. In this article, we will take you on a journey through time, exploring the origins, techniques, and impact of phreaking in the United Kingdom. From the early discovery of the “toll a drop back” technique to the organized networks of British phreaks, we will delve into the intricacies of this underground subculture and its interaction with the British telecommunications system. Join us as we uncover the untold stories and shed light on the evolution of phreaking in the United Kingdom.

Early Days: Unveiling the “Toll A Drop Back” Technique

Phreaking in the United Kingdom dates back to the early fifties when a groundbreaking technique known as the “toll a drop back” was discovered. At the heart of this technique was the Toll A exchange, located near St. Paul’s, which served as a crucial hub for routing calls between London and neighboring non-London exchanges. The ingenious trick employed by phreakers was to dial an unallocated number and quickly depress the receiver-rest for half a second. This action triggered the “clear forward” signal, granting the caller an open line into the toll exchange. Leveraging this newfound access, phreakers could dial 018, connecting them to the trunk exchange—the first long-distance exchange in Britain—without incurring any additional charges. The discovery of the “toll a drop back” technique marked a significant milestone in the history of British phreaking, paving the way for further exploration of telecommunication systems.

Unveiling the Signaling System: Breaking the Code

To fully comprehend the achievements of British phreaks, it is crucial to understand the signaling system employed by the UK network. Signaling System No. 3, as published in the “Institution of Post Office Engineers Journal” and reprinted in the Sunday Times (15 Oct. 1972), utilized pairs of frequencies chosen from six tones separated by 120Hz. Armed with this knowledge, resourceful phreaks devised their own devices known as “bleepers” (equivalent to “blue boxes” in the US) by leveraging different MF tones than their American counterparts. Consequently, US “blue boxes” would not function in the UK without modifying the frequencies. This unique adaptation showcased the ingenuity and adaptability of British phreaks in navigating the intricacies of the telecommunications system.

The Shift to Simpler Systems: Exploring the Pulse-Based Technique

In the early seventies, a simpler system emerged in the form of pulse-based techniques employing a consistent frequency of 2280Hz. For more in-depth information on this subject, we recommend referring to Atkinson’s “Telephony and Systems Technology.” These pulse-based methods provided an alternative avenue for phreakers to explore, expanding their repertoire of techniques within the British telecommunications landscape. As technology advanced, so did the methods employed by phreaks, reflecting their insatiable curiosity and relentless pursuit of knowledge.

Organizing the Network: From Cambridge to Undercurrents

During the early days of British phreaking, the renowned Cambridge University Titan computer played a pivotal role in recording and disseminating the numbers discovered through exhaustive dialing of local networks. These numbers formed the foundation of a vast network of links, connecting local exchanges across the country while bypassing traditional trunk circuits. However, due to variations between the internal routing codes used by the UK network and those dialed by the caller, phreaks had to rely on “probe and listen” techniques, akin to scanning in the US, to discover these codes. By inserting likely signals and carefully listening for successful outcomes, phreaks gradually uncovered the secrets of the British telecommunications system. The results of their scanning endeavors were then circulated among fellow phreaks, facilitating the sharing of invaluable information, including new numbers and equipment. The comprehensive repository of knowledge they amassed came to be known as “Undercurrents,” the intricate map that enabled British phreaks to thrive and collaborate.

Understanding the Three Layers: Local, Trunk, and International

To grasp the extent of British phreaking exploits, it is essential to conceptualize the UK phone network in three layers: local, trunk, and international. Subscriber Trunk Dialing (STD) in the UK served as the mechanism to elevate calls from local lines to trunk or international levels, providing legitimate means of communication. British phreaks, however, sought to explore the possibilities offered by the trunk level. They realized that by deciphering the right routing codes and utilizing them correctly, a call at the trunk level could be routed through numerous exchanges, transcending geographical boundaries. This revelation spurred their relentless pursuit of unlocking the secrets that lay within the telecommunication infrastructure. Furthermore, they sought ways to transition from the local to trunk level without incurring charges, devising innovative methods such as bleeper boxes. While chaining—a technique involving long strings of digits—was an option, it presented challenges, including fading speech quality as the chain lengthened. British phreaks preferred gaining access to trunks rather than relying on chaining, which led them down paths of discovery that revealed both the vulnerabilities and ingenuity of the British telecommunications system.

The Fascinating World of Fiddling: Rewiring Exchanges for Advantage

Phreaking in the UK also entailed uncovering instances of “fiddling” within the telecommunication infrastructure. Fiddling referred to instances where engineers modified exchanges to gain personal advantages. The relatively prevalent step-by-step (SXS) electromechanical exchanges installed in Britain during the 1970s provided a relatively simple environment for such manipulations. Notably, even the Canadian system harbored a backdoor on a 4A CO exchange, which British phreaks attempted to exploit through scanning three-digit exchanges such as dialing 999, 998, 997, and listening for the distinctive “beep-kerchink” sound. By identifying three-digit codes that allowed direct access to a tandem in the local exchange, phreaks could bypass Automatic Message Accounting (AMA), ensuring they wouldn’t be billed for their activities or rely on the 2600Hz tone. The revelation of such vulnerabilities demonstrated the resourcefulness and tenacity of British phreaks in their quest for knowledge and exploration.

Bridging the Gap: Phreaking and the General Public

Phreaking in the UK during the sixties saw an interesting phenomenon where “ordinary” individuals turned to occasional phreaking out of frustration with the inefficient operator-controlled trunk system. This surge in phreaking activity coincided with a strike in 1961, which left operators unreachable. Exploiting the habit of operators to repeat codes as they dialed requested numbers, people quickly learned the numbers they frequently dialed. The only requirement was knowledge of which exchanges allowed access to the trunk network to relay the desired number. Quiet environments became essential for these activities as timing relative to clicks played a crucial role. These “ordinary” phreakers epitomized the shared sentiment of individuals seeking alternatives within an imperfect system.

The Old Bailey Trial: A Glimpse into the World of British Phreaks

The most famous trial involving British phreaks took place at the Old Bailey, commencing on October 3, 1973. The trial shed light on the modus operandi of phreakers, revealing their ingenious methods. By dialing a spare number at a local call rate, which involved accessing a trunk to another exchange, phreakers would send a “clear forward” signal to their local exchange, indicating that the call had concluded. However, the distant exchange remained unaware of the call’s completion, as the caller’s phone remained off the hook. With an open line into the distant trunk exchange, phreakers would send a “seize” signal of ‘1’, granting them access to its outgoing lines. Armed with the correct codes, the world was theirs to explore. While the actions of the phreaks might have seemed illicit to some, others, including the judge presiding over the trial, viewed phreaking as a hobby—a scientific pursuit conducted by enthusiasts seeking to understand and test the limits of the world’s largest computer, the global telephone system.


In conclusion, the history of phreaking in the Kingdom of Great Britain is a captivating tale of exploration, ingenuity, and the relentless pursuit of knowledge within the realm of telecommunication systems. From the early discovery of the “toll a drop back” technique to the organized networks of British phreaks, this underground subculture played a significant role in shaping the understanding of telecommunication systems and their vulnerabilities. The stories of British phreakers highlight the power of curiosity, collaboration, and the ceaseless quest for understanding the ever-evolving world of technology. As we reflect on the achievements of these trailblazers, we gain valuable insights into the broader landscape of telecommunication security and the intricate relationship between technology and human ingenuity.

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