7. The phone numbering system

So, i had some close friends telling me the other day that they were confused in regards to the phone numbers. I'll try to explain it as best as i can. This post tries to answer questions like "what is an NNX" or "what is an NPA" or even the generic "how a phone call is made"  ?

Basically, at your home or for your cell phone you have a phone number. The phone number is more or less like an address that woull tell a greather authority who is located where. Let's take an example

  • The phone number 001-416-123-1234 would tell your phone company that it should direct this call to the authority (like a master database) that handles North America (it detects the right number of digits and the begining code of 001 tells it it's a North America number).
  • Once it reaches North America, the authority looks in its database to see what provider has 416 numbers (they are also refered to as NPA - Numbering Plan Area - aka Area code).
  • Once it finds that authority it asks it about the next 3 digits (123 - also called an NNX - Numeric Numbering eXchange) to find out the provider for that range.
  • At this point let's say your provider is called At&t or Bell knows that they have to connect you with the local phone number 1234. You might ask how does Bell or At&t know who has extension 1234 ? Well, they maintain a database and even though they refer to the number as 416-123-1234, the system actually understands only a specific number a digits. Most of the time are the last 4 (we'll cover this later). See how your phone number goes from a 10 digit phone number to a 4 digit ? You can find more info if you really want to here on Wikipedia
  • So looking at our phone number of 001-416-123-1234 we can generalise numbers for North America as being formed from:
    • Intl(1 or 3 digits depending or where you initiate the call from ... 1 from NA, 3 from europe)
    • NPA (3 digits) - aka as area code
    • NNX (3 digits) - aka exchange - tells you who provides that phone number with service from an operator point of view
    • Phone number (4 digits) - your phone number.
  • All the numbers brought together form what we call a DID (or a Direct Inward Dialing). This tells you that someone can contact you directly by dialing the number from anywhere in the world.

How about the reversed process where you make a call ? Well, there are two scenarios.

  1. Let's take the most simple one. You have a DID or your company bought 10 DIDs for 10 stations and assigned them on a 1 for 1 basis. You pick up the receiver and make a call. The provider (can be At&t or Bell) will reconstruct your DID and present that number to the first autority that it will have to reach. Nothing more than reverse engineering for what we were speaking about earlier.
  2. Because of financial reasons (kind of expensive to buy 100 numbers when you have 100 employees), companies decide to buy only a few DID and assign them on the groups rather than the particular people. A company that chooses a 4 digit dialplan (how many digits the extensions have inside) can assign any particular extension from the 1000 - 9999 range. However, when making a call from a station that does not have a DID is proper programmins is not done, nobody can call the number back.
    1. Here is a Scenario. Company 1 buys 10 DID's (let's assume 416-123-1000  to 416-123-1009). The company however has 20 employees. If they program the system in a wrong way (they chose to recreate the DID not let the provider do it), the employee with extension 1012 would present a number of 416-123-1012 when calling someone. If that person decides to return the call, the call would land somewhere else, as soon as the number 1012 is requested from NNX 123, that autority would forward that call to the rigth owner of that particular DID. It's like you would send an envelope but put the neighbour's address as the sender. When the postmail will receive an answer to that mail it would direct it to the address that you put on, not to the address that originated the mail. So what can we do in this case ? The solution is simple. We can change (will see how) the reconstruction rule to fake the number putting a rule in that would say if extensions from the numbers 1011-1019 would make an outside call, that call would be presented as 416-123-1000 (assuming in this case that this is the reception). However that person can take incoming internal calls because you are the own authority for that number. If 1003 wants to call 1012 it would ask your own PBX (Avaya in this case) and the PBX would know when 1012 is directly without asking any other authority. Would connect the call automatically. This is why sometimes you return a call and you get the reception. The person that originally called you did not have a DID.

I hope that you now understand how the phone numbers are formed.

Things to remember...

  1. Not all extensions are DID's
  2. The process of adding the authorities in our system in called a dialplan.

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