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Friday 22 January 2016

The Phone Phreak

I dialled ‘93490’ and then my parents’ number and was astonished to hear it ring. You were supposed to dial ‘0405’ from Leeds to connect through to their exchange, but I had used ‘93490’. I would get more time for my money. It would be charged as a local call rather than long-distance. I pressed my 2p into the slot[1] knowing I had just got the better of the Post Office Telephone system. I was now a ‘phone phreak’.

Phone phreaking had its zenith in nineteen-sixties America and was the precursor of computer hacking. Among its devotees were Steve Jobs and Steve Wozniak, the future founders of Apple. As this was long before home computers and even longer before the internet, they satisfied their curiosity by fiddling with the telephone network.

When touch-tone phones were introduced in America in 1963, it was discovered almost by accident that whistles of specific frequencies could be used to control the electronic relays in telephone exchanges to get free calls, including long-distance and international calls. Phone phreaks were soon building tone and signal generators to make things easier. There were the so-called ‘blue boxes’ which gave free outgoing calls, ‘black boxes’ which received incoming calls without charging the caller, and ‘red boxes’ which tricked pay phones into registering the insertion of non-existent coins.[2]

Unfortunately, these devices did not work in the U.K. We still had telephones which communicated with the exchange through electronic clicks or pulses generated mechanically by rotary dials. Even when push-button phones began to appear in the nineteen-seventies, they were pulse phones rather than touch-tone. Our telephone exchanges were still based on electro-mechanical relays rather than today’s electronic switching systems.[3] In other words, the U.K. telephone system did not offer much scope for phone phreaking activities, which was a pity because from Leeds it cost what to me seemed a fortune to keep in touch with home.

What I had done in making my ‘93490’ call was ‘chaining’ (i.e. linking telephone exchanges together) rather than phreaking. To understand this we need to go into a little bit of detail. Skip to the next diagram to avoid it.

In 1972, telephone calls were charged in three different bands: (A) the local rate for up to around 35 miles, (B) 35-50 miles and (C) over 50 miles. Distance was measured between exchanges. Calls from Leeds to my parents’ exchange were in band B. This meant that after six in the evening, calling home from a phone box in Leeds cost 2p for 45 seconds, which was twice the local rate of 2p for 90 seconds. To put this in context, a five minute call home cost more than a lunchtime sandwich; money I could ill-afford to spare out of the wretched salary I received as an articled clerk. To be able to get twice the time for the same price was a welcome saving. Even better was the satisfaction of beating the system.

Chaining became possible when subscriber trunk dialling (STD) was introduced to allow callers to dial numbers themselves rather than waiting to be connected by a telephone operator. One of the first steps was to amalgamate area and sub-area exchanges so that, for example, if you were on the Goole-Thorne exchange, and you wanted to make a local call to a number in the Rawcliffe sub-area, you began your dialling with ‘83’ and were switched through automatically. The system then allowed the same technique for calls between adjacent main exchanges, also charged at the local rate. So to make a local call from the Pontefract exchange to a number on the Goole-Thorne exchange, you began with ‘90’ (you could have used the national STD code ‘0405’ instead, but this would have been charged at a higher rate). Furthermore, and this is the important bit, Pontefract callers could make local calls to Rawcliffe numbers by passing through the main Goole-Thorne exchange using the code ‘90-83’. Exchanges were wired to allow codes to be chained together. It was not explicit, you simply looked up the required dialling codes on a list, but this is what you were doing.

One side-effect of this was that you could pass straight through an exchange, in at one side and out at the other. So if you wanted to make a local call from Goole to Pontefract you would begin with ‘93’, and if you wanted to make a local call from Pontefract to Leeds you would use ‘92’, and if you put them together you could pass straight through Pontefract and call Leeds numbers from Goole with ‘93-92’ instead of the official ‘0532’. The difference was that you were charged at the local band A rate instead of the more expensive band B.

In the opposite direction, Pontefract from Leeds was ‘934’, and the Goole-Thorne exchange from Pontefract was ‘90’, so if you dialled a Goole-Thorne number from Leeds using ‘934-90’ instead of the official ‘0405’ it was charged as local call. This is what I did when I called home from the call box in Leeds.

It looked like this:

STD subscriber trunk dialling codes Leeds Pontefract Goole Scunthorpe

It would therefore have been possible to call Rawcliffe numbers and Scunthorpe numbers at local rates from Leeds by passing through the Pontefract and Goole exchanges using the codes ’934-90-83’ and ’934-90-95’ respectively.

In theory there was no limit to the number of links in the chain. For example, you could make a ‘local’ call from Bradford to Hull via the Leeds, Pontefract, Selby and Howden exchanges using ‘92-934-91-96-93’, and from Scunthorpe to Ripon via the Goole, Pontefract, Leeds and Harrogate exchanges using  ‘95-93-92-92-91’. The main problem was that the volume became fainter as the chain grew longer.

As I travelled around for work, I spent several months collecting dialling codes from different areas, sometimes even copying them down in call boxes. I began to construct a network map which recently turned up among old papers in my capacious loft:

STD subscriber trunk dialling codes West Yorkshire

It would have been great to have been able to get all the way through to friends at university in London, but as the map shows, I didn’t get any nearer than Retford, Worksop and Chesterfield. In part, collecting and organising large numbers of telephone codes was an enormous and tedious task, and even I began to lose interest. A glance at my notes for the Leeds and Pontefract exchanges, below, indicates how time-consuming it was. In addition, codes were changing all the time. The network was gradually being improved and eventually all dialling codes were changed to the present-day 0--- and 0-- format, which prevented chaining. Within two or three years my cheap call codes no longer worked.

STD subscriber trunk dialling codes Leeds Pontefract

It was fascinating at the time. One insight was how the STD dialling codes were based on earlier alphabetical codes, a legacy we still have today. Leeds was once ‘LE2’, and other locations beginning with ‘LE’ were Ledbury ‘LE1’ and Leicester ‘LE3’. So the STD code for Leeds became ‘0’ (for operator) ‘LE2’ (for Leeds), which translates on the letter-number dial into ‘0532’. Similarly, Ledbury became ‘0531’ and Leicester ‘0533’. Hull’s ‘HU2’ became ‘0482’, Huddersfield’s ‘HU4’ became ‘0484’ and Goole-Thorne’s ‘GO5’ became ‘0405’. There were lots of exceptions and inconsistencies, and many codes have since changed, but in many cases the legacy still persists. Goole, for example, is now ‘01405’, and Hull is ‘01482’.

There was one last occasion I remember cheating the system. I moved into new accommodation to be told, almost immediately, that the building was to be renovated and all tenants had to be out by the end of the month. Off the shared kitchen was a telephone with a dial lock to prevent unauthorised use. Knowing you could bypass this by ‘switch-hooking’, i.e. rapidly pressing and releasing the cradle to simulate the pulses generated by the rotary dial,[4] I tapped out a call to my friend Hugo in Belgium. Clicking at the required six or seven times per second is not easy, but I managed the ‘01032’ international code followed by Hugo’s number at the first attempt, and we spoke for a good ten minutes. I bet all hell was let loose when the bill came listing an expensive international call, but I still have no regrets about it. The rent was excessive and it was a pain having to find somewhere else to live so soon after moving in, at such short notice.


[1] By 1972 most public telephone boxes had been converted to the Pay-On-Answer type which accepted coins only when a call was answered. These replaced the older Button ‘A’ and ‘B’ boxes which required payment in advance. In the older boxes you fed coins into a slot, dialled the required number, and when someone answered you pressed Button A in order to be heard and your coin dropped irretrievably further into the machine. If there was no answer you pressed Button B to get your money back. When your time was about to run out you heard a series of urgent ‘pips’ and if you failed to feed in further coins you were summarily cut off. To circumvent this you could feed in lots of coins at the start and Button B would return any unused coins at the end. As children, we rarely passed a phone box without going in to press Button B in case someone had forgotten to retrieve their unused coins.

[2] For example, the black box worked by altering the electrical potential on the line so as to trick the telephone exchange into registering that the phone was still ringing when in fact you had answered it and were happily chatting away. Details of this and other techniques can be found at, which is a repository of old phreaking documents written by people far more knowledgeable than me. One of the files gives a history of British phreaking. Don’t go there if you find my blog post too technical. However, the site home page provides links to an Aladdin’s cave of all kinds of old files. 

[3] Even today, telephone networks can still accept pulse telephones. A year or so ago our phone was deadened by a power cut and we had no working mobile (cell) phone in the house. We do however keep an old push-button pulse phone with the emergency candles and camping stove. Because pulse telephones are powered by the small electric current that comes down the telephone wires, I was able to use it to contact the supply company to find out how long the power cut was likely to last. This is also why people who misused the telephone system in the nineteen-sixties and nineteen-seventies, such as anyone caught chaining, were charged with the heinous crime of wasting Post Office electricity.

[4] It is said that by switch-hooking the old Button A/B telephone boxes you could make calls without paying for them at all. I never tried this and don’t know whether it is actually true. One claim that it is appears at the end of at where there are also some great photographs of telephone boxes and Button A/B telephones.

Saturday 2 January 2016

Jumped-Down Catholics

This will be reposted as a New Month Old Post on 1st August 2023