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Telephone Numbering Plans

Understanding Telephone Numbering

Although far from perfect, global telecommunication works. We type a number into our telephone keypad, usually press a “call” button, and magically, within moments we’re connected to the person we called. It works the same whether we’re dialling our neighbor’s home phone, a VoIP number (Voice over Internet Protocol, like Skype), a cell phone or a person half-way around the world using almost any type of telecommunications system.

To make this possible, telephone numbers need to be (at the very least):

  • Standardized
  • Unique

Telephone numbers being unique seems obvious. If I call 0861262275 then I expect to be connected to Intertel’s support center without exception. I don’t expect to have my calls answered by Jimmy’s Pizza today and then Mike’s Mechanical Services tomorrow. Uniqueness is ensured by each telecommunications company who have controls in place to ensure that once a number is assigned to a customer, it is locked and not available for assignment to another customer. But what stops Cell C, for example, issuing a telephone number that overlaps with a customer of another mobile operator, like Telkom Mobile, who both have numbers in the 074 range?

This is where Standardization starts to play a role. You should already know that when calling a foreign country you need to begin the call with an international access number (in South Africa, that is 00) and then following that one needs to dial the international country code of the country that is being called. Say I wanted to call the UK, the number would begin with 0044. After that the number needs an area code or service code. If i’m calling a friend in London (which has the area code 020) then the number would have the following six digits at the beginning: 004420. You’ll notice that the first 0 from the London area code was dropped. That must be done as the 0 is only used when calling a number from the same country. The remaining numbers are the locally assigned (unique) telephone number for the customer or device that you are calling. When people overseas call South Africa, they’d need to dial an international access code, followed by the South African country code (which is 27), then the service code (without the leading 0), and then the number. For example, calling Intertel’s Sandton number (which is 0110835468) from the United States, a person would dial 01127110835468 because 011 is the USA’s international access number, 27 is South Africa’s country code, 11 is the area code for Sandton when called from another country and the rest is our telephone number.

How does the calling phone know where the Sandton telephone is situated, or doesn’t it need to know? The simple answer is that it doesn’t need to know when dialling the number. In much the same way as the telephone number was constructed from an international access code, a country code, a service/area code and a unique number, the call itself is routed through a similar process. When 011 is dialled, the telephone is connected to an international calling service which is expecting the country code to be typed so that it knows which country’s incoming international service to connect the call to. When it receives the 27 from the caller, the call is routed to South Africa and connected to a similar system that waits for the next number (or set of numbers) dialled. Based on what it receives, the call is routed to yet another exchange until it arrives at the exchange that services the Sandton office and the final digits are received. Only that node in the communications network really needs to know that 0835468 is Intertel’s number. At every other point in the journey, all that the one node needs to know is which node connected the call to it, and which node it must hand the call over to. This is where standardization is critical. The way in which numbers are assigned to service types and areas allows enables a call to find its way to the destination without everyone from the caller, to every node in between having a global telephone directory available.

How numbers are assigned is becoming ever more important as the number of communication service users increases. The current standard for telephone number length (in South Africa, that is) is 10 digits. There are also short code numbers of less than 10 digits, but we’ll deal with those later on. The first three digits identify what type of telephone service a number is assigned to (like 086 for the Intertel support center which is a non-geographical number) or what area the number is assigned to (if the number is a geographical number, like the Sandon number, 011). Other information can often be inferred from the first three digits, such as which telecommunications company is responsible for the provision of that telephony service (sometimes this requires a fourth and fifth digit and then one must not forget about phone number portability which lets you take your 083 number from MTN and use it with Vodacom). South Africa’s telephone number range is under the authority of the Department of Communications and is administered by ICASA (the Independent Communications Authority of South Africa). As in other countries, South Africa has a formal numbering plan. This plan describes how numbers are formulated, assigned and used, and using the information within that plan, one is able to distinguish one type of number from another. Lets go through it…

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