How can a phone number be four numbers?

A complete phone number is unlikely to be just four digits long, but the local part of a phone number without an area code could be that short in rural areas of some countries. (For example, the rural community of Brampton in the United Kingdom has four digit phone numbers which can be dialled as four digits locally - but when the six digit area code is included, the overall length is ten digits.)

This is usually an efficiency measure: in many parts of the world, the length of a subscriber's phone number varies according to local demand, as a way of making efficient use of the numbering space.

A big city like London might have eight-digit phone numbers locally, allowing a theoretical maximum of 100,000,000 numbers. However, a very small town might easily be served with only 10,000 numbers, which would easily be accommodated by a four digit number.

Using variable length phone numbers and area codes is a good way of not wasting spare capacity, as several small towns can be accommodated in the amount of space that one large city might use.

Example

Think of how you could divide up a batch of 10-digit numbers beginning with 200 (i.e. 200xxxxxxx).

Option 1:

You could use all the numbers for a large city.
(200) 000 0000 to 999 9999

That uses 7-digit local phone numbers, giving 10 million possible numbers. A short, 3-digit area code is used to keep the overall national number length down to 10.

Option 2:

You could split up the same batch of numbers between several areas which need fewer numbers each:

(2000) 000 000 to 999 999 for Town A
(2001) 000 000 to 999 999 for Town B
- 6 digit local numbers, allowing 1 million combinations

(200231) 0000 to 9999 for Village C
(200232) 0000 to 9999 for Village D
- 4 digit local numbers, allowing 10,000 combinations

etc...