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Myths and Misunderstandings

This is a collection of the myths and misunderstandings of the telecommunication domain.

A telephone number is not a number!

The term "telephone number" is so common that it is easy to forget that it is just a term and not an accurate description. As a telephone user, this is usually not important, but as soon as you need to process it in some way, such as programming some software, not realising it is a number will get you into trouble. Let's see what can go wrong if you naively treat a telephone number as a number:
  • Many telephone numbers start with a zero and can be up to 15 digits in length. Depending on your programming tool of choice, one of the following might happen:
    • The zero is ignored. Converting to an internal binary number and back to a human readable form will loose the zero.
    • The zero taken to be an octal indicator - either it will be stored incorrectly, or, if it contains a digit above 7, not stored at all or stored as zero.
    • The "number" is too big for the storage and only 5 (16-bit storage) or 10 (32-bit storage) digits are stored.
  • A number may contain non-numeric digits. Telephone keypads can compose "numbers" containing a hash (sometimes called the square, or pound key in the US), the star (a.k.a. asterisk). Internal to the telephone network, the digits A, B, C, and D are also possible.
  • Telephone numbers for human comprehension often include spaces, hyphens, or parentheses to make them more readable. International numbers are usually preceded by +.
  • The structure of telephone numbers varies between countries and areas. The North American numbering area uses a fixed format of 3-3-4, normally separated by hyphens - almost all of the rest of the world does not.
Of course, not all of these may be of concern. If programming within the telephone network you don't need to consider the various formats that humans write numbers, but you do need to know that A, B, C and D may be valid digits.

A telephone number does not address a telephone

  • The device at the end of the line might not be telephone. It could be a fax machine or modem. Devices within the telephone network often have an address or "number", but they cannot be dialled as if they were a telephone. They might not even know about telephone calls, for example, a SMSC (Short Message Service Centre).
  • A number might address another network, rather than a device. The most common example is calling a company switchboard.
  • The number might be a routing service. Examples are number translation (route a premium rate number to a geographic number), or call centre routing.

Number type and plan

If you are just a telephone user, or deal with telephone numbers outside the telephone network, for example, a customer database, then most of the following is probably not important. However, if you have to deal with the telephone signalling network, then it is critical to understand this.

A number has a plan. The most common, and the only one I will cover here, is the PSTN (Public Switched Telephone Network) defined by ITU-T I.164. Other examples are Telex and Maritime Mobile. The plan determines how the rest of the number is interpreted.

An I.164 number has a type, for example:

  • International. This means it starts with a 1-3 digit country code. E.g. 441232123456. This is the only valid type for sending between networks and between countries.
  • National. The number is only valid within a country. E.g. 1232123456.
  • Unknown. The number has not yet been decoded (or previous attempts to decode have failed). E.g. 01232123456 or 00441232123456.
Note: In the UK, the national access code is 0, the international access code is 00, and the country code is 44.

You may have missed the subtlety here: 00441232123456 is of unknown number type within the telephone network, it is not an international number. Once decoded, it may become type=International number=441232123456. An unknown number can only be decoded if the locality in which it originated is known: Taking 00441232123456 again, within the UK this can be decoded because the international prefix is known (00), try to decode this same number in North America and it will fail because the international prefix there is 010. Generally, numbers need to be decoded and converted to international type in the network in which it originated.

Low Cost International Calling

Number Withheld

Number Withheld is a misnomer. Internal to the telecoms network it is a flag called "Presentation Restricted". It means that the number must not be displayed to the end user, what is does not mean is that the number is not sent across the network. This means:
  • The number is contained in all signalling messages in the telephone network, and will appear in any logs stored by the network operator. Normally these telephone logs are only used for billing, but they are also available for malicious call tracing and criminal investigations. Number Withheld is not a barrier to preventing abusive or malicious phone calls - in fact, it helps because it forces the victims to contact the police or telephone company instead of just ignoring it and hoping it will go away.
  • Certain end users are exempt from the "Presentation Restricted". The obvious example is the emergency services call centre (e.g. 112, 999 and 911).
  • Although end users are not allowed to know the number, their telephone company does. An application of this is malicious call barring: The end user on receiving a malicious call enters a pre-arranged code on their keypad - this is detected by the telephone network and the number is added to a blacklist by the telephone company, so even though the end user does not know the number they can prevent further calls from that number. This can be used by call centres to protect their staff from repeated abusive calls.
  • The Presentation Restricted flag relies on the destination network to honour it and not present it to the person called.

Misuse of the Number Withheld facility

Last Updated: Site 2007-10-11 Page 2006-01-17

© Copyright Mark Easterbrook 2005-2006.