Falsehoods Programmers Believe About Phone Numbers

... and tips on how to use libphonenumber.

Given how ubiquitous phone numbers are and how long they‘ve been around, it’s surprising how many false assumptions programmers continue to make about them.

1. A phone number uniquely identifies an individual

   It wasn‘t even that long ago that mobile phones didn’t exist, and it was common for an entire household to share one fixed-line telephone number. In some parts of the world, this is still true, and relatives (or even friends) share a single phone number.

2. An individual has only one phone number

   Obviously, this isn't necessarily true.

3. Phone numbers cannot be re-used

   Old phone numbers are recycled and get reassigned to other people.

4. Phone numbers that are valid today will always be valid. Phone numbers of a certain type today (e.g., mobile) will never be reassigned to another type.

   A phone number which connects today may be disconnected tomorrow. A number which is free to call today may cost money to call tomorrow. The phone company may decide to expand the range of available phone numbers by inserting a digit into an existing number.

   Tip: Don’t store properties for a phone number such as validity or type. Check this information again from the library when you need it.

5. Each country calling code corresponds to exactly one country

   The USA, Canada, and several Caribbean islands share the country calling code +1. Russia and Kazakhstan share +7. These are not the only examples!

6. Each country has only one country calling code

   As of this present moment (in Mar. 2016), phones in the disputed territory and partially recognised state of Kosovo may be reached by dialing the country calling code for Serbia (+381), Slovenia (+386), or Monaco (+377), depending on where and when one obtained the number.

   Tip: Use a phone widget to encourage users to enter their phone number in an international format such that it can be unambiguously understood.

7. A phone number is dialable from anywhere

   Some numbers can only be dialed within the country. Some can only be dialled from within a subset of countries, such as the international 00800 numbers. Some may be dialable only if the caller is a subscriber to a particular telecom company.

8. There are only two ways to dial a phone number: domestically and from overseas

   Some numbers may need different prefixes depending on: the carrier you are using; what device you are dialling from/to; whether you are inside or outside a particular geographical region.

   Examples:

   • In Brazil, to dial numbers internally but across a certain geographical boundary, a carrier code must be explicitly dialed to say which carrier you will use to pay for the call.
   • In Nepal, the leading zero in national format is omitted depending on whether the originating phone is mobile or fixed-line.
   • In New Zealand, you need to dial the area-code (e.g. 03) even if the number is within the same area-code region as you are, unless it is “close” (something approximating city/district boundaries), in which case it shouldn’t be dialled.

   Tip: Use formatForMobileDialling to get the number a user should actually dial on their mobile phone.

9. To make a number dialable, you only need to change the prefix

   In Argentina, to dial a mobile number domestically, the digits “15” need to be inserted after the area code but before the local number, and the “9” after the country code (54) needs to be removed. This transforms +54 9 2982 123456 into 02982 15 123456.

10. No prefix of a valid phone number can be a valid phone number

    In some countries, it's possible to connect to a different endpoint by dialing more digits after a number. So “12345678” may not reach the same person as dialing “123456”.

11. An invalid number will not reach an endpoint

    In some countries, or on some phones, extra digits are thrown away. Hence, 1-800-MICROSOFT is an invalid number - but it still connects to Microsoft, since any later digits are ignored. Numbers such as “911” can be reached by dialling “911 123” in some countries: but not in others.

    In other countries, invalid numbers may be “fixed” by a carrier, e.g., adding a mobile token if they know it is a mobile number, such that it connects.

12. All valid phone numbers follow the ITU specifications

    ITU-T specifies that a phone number cannot be longer than fifteen digits, with one to three digits reserved for the country calling code, but valid numbers in Germany have been assigned that are longer than this.

13. All valid phone numbers belong to a country

    There are many “country calling codes” issued to non-geographical entities, such as “800” or satellite services.

14. Phone numbers contain only digits

    In Israel, certain advertising numbers start with a *.

15. Phone numbers are always written in ASCII

    In Egypt, it is common for phone numbers to be written in native digits.