How to format international telephone numbers

Country codes

The most important part to remember is to add the international country code. A mobile phone number in the United Kingdom always starts with the digits '07', for instance ‘07911 123456’. If you don't add an international country code, a computer might interpret that phone number as something else altogether. The country code for Russia is ‘+7’, and Russian mobile phone numbers start with ‘+79’, so ‘+7911 123456’ might actually be a Russian mobile phone number.

Adding ‘+44’, the UK’s country code makes the number unambiguous; ‘+44 7911 123456’ will always connect to a mobile phone in the UK.

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E.164 formatting

It might be clever to format numbers using the E.164 recommendation. An international E.164 number is designed to include all of the necessary information to successfully route a call or SMS message to an individual subscriber on a national public telephone network.

Here's how the E.164 numbering plan works:

• A telephone number can have a maximum of 15 digits
• The first part of the telephone number is the country code (one to three digits)
• The second part is the national destination code (NDC)
• The last part is the subscriber number (SN)
• The NDC and SN together are collectively called the national (significant) number

To use the previous example:

• Country code: +44
• National destination code: 7911
• Subscriber number: 123456
• The result: +447911123456

Two important things to note: First of all, in the international E.164 notation a leading ‘0’ is removed. The UK mobile phone number ‘07911 123456’ in international format is ‘+44 7911 123456’, so without the first zero. Secondly in the E.164 notation all spaces, dashes [‘-‘] and parentheses [ ‘(‘ and ‘)’] are removed, besides the leading ‘+’ all characters should be numeric.   

International phone number validator

A great way to check whether a phone number is correct, or how it should look in E.164 formatting, is by using the Phone Number Parser demo. This is part of a piece of software that Google uses to check phone number formatting in Android phones.

The website ‘howstuffworks’ has more valuable background information on the origin of the International Telecommunication Unit (ITU) and the numbering plan E.164. 

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