Ringing the Right Number
Name: Jennifer G. W.
Date: September 2003
When someone dials your phone number how does it ring the correct phone?
A telephone number looks like (ABC) - DEF - HIJK Here I use each letter to stand for each of
the numbers 0,1,2,3,4,5,6,7,8,9
(ABC) is called the area code: It can have values starting with 000, 001, all the way up to
DEF is called the exchange: It too can have values starting with 000, 001, all the way up to
HIJK is a number that can have a value starting with 0000, 0001, 0002, all the way up to
This is simplified because some combinations are not used for various reasons. For example,
no one has the number (000)-000-0000 or (999)-999-9999 and some special short-cut numbers like
the emergency number '911'. The area code ABC is just like the name sounds. It marks off a
geographical area. In a crowded city there may be five or six area codes, but in an uncrowded
that number may cover many miles.
Once you dial the first three numbers, your phone is connected to the area covered by that area
code. Inside each area code there are several exchange numbers. What I have called DEF above.
Not every possible combination of numbers is used but all the possibilities would be from DEF
equal to 000, 001, 002 up to DEF equal to 999.
Now your phone is connected to the area code and to an exchange inside that area code.
Inside a given exchange number, there could be possible numbers HIJK starting with 0000, 0001,
0002 all the way up to 9999.
Then every phone has its own number. You can see how this lets the phone company use numbers
over and over.
For example: 212-666-6666 would be different from 222-666-6666 because the area codes are
212-666-6666 would be different from 212-676-6666 the same area code but a different exchange
within the same area code.
And 212-676-6666 would be different from 212-676-6667 -- The two are the same except for the
final 6 or 7. This adds up to a lot of possible combinations even when the "special numbers"
are not used.
There is a wire going from your phone to an "exchange" where it can be connected to any of the
other phones which are connected to wires going to that exchange. There are a lot of phones,
so it looks very complicated, but the basic idea is simple.
If there were just 10 phones connected to an exchange, then someone could dial just one number
(0, 1, 2, 3, 4, 5, 6, 7, 8, or 9) to connect to any of the phones. For example, if your phone
were connected to line 3, someone would just have to dial 3 to connect to your phone.
It is more complicated because a ring is needed and the exchange has to know when a phone is
picked up, must detect the number dialed, and must detect when phones are hung up.
One exchange can handle up to 10,000 phones with numbers 0 to 9999. The last four digits of
your phone number are your number in the exchange. The other six (the area code for long
distance dialing and the exchange) allow anyone in the country to call you. More digits are
needed for international calling.
For example, the number (212) 123-4567 would be a phone numbered 4567 in the New York City
area (area code 212) and the exchange identified by 123.
Now, of course, the switching is handled by computers rather than simple switches, but the
ideas are still the same.
Best, Dick Plano...
In a modern phone system, when you pick up the phone, and electronic switch senses that you
are on the phone and you hear a dial tone so that you know that your phone is working. You
then dial the number using the touch-tone keypad. The telephone signal is sent to the area
network that you are dialing and the telephone call is completed to the number that you
dialed. I hope that this is helpful.
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Update: June 2012