To call London (or in fact any UK number) from overseas, you first need to know the International Direct Dialling code (IDD) for your country.
You can find these codes here. Examples are 011 for the United States or 0011 for Australia.
Top Tip: On a mobile phone, long-press the zero key for a few seconds to bring up the + symbol on the numeric keypad, and this will automatically insert the IDD of the country you are currently in.
This makes things a lot easier because in each country the international access code is different.
Then all you have to do is enter the country code followed by the number.
In this example, we will look at calling the London fixed-line number
020 7123 4567
To call this number you need to enter the following:
[International Direct Dialling code] +
[UK country code 44] +
[UK number without the leading zero ie 20 7123 4567]
Note that when calling international numbers you always DROP the leading zero of the local number you wish to dial.
In the above example, the country code shown is 44 for the UK. The country code is different for every country.
A list of country codes can be found here.
If you were calling this London number from the United States then you would dial
011 44 207123 4567
IDD (for US) +
UK COUNTRY CODE 44 +
LOCAL NUMBER (less the leading zero)
OR from your mobile, simply enter +44 207123 4567
Mobile numbers
For a UK mobile number, these always start with 07, and may be written in the following ways:
07881 234 567 or +44 (0) 7881 234 567 or +44 7881 234 567 (the full international number)
Note the + symbol means you have to add (+) your International Direct Dialling code (IDD) before the number.
For example, to call the UK mobile number 07881 234 567 from the US you would dial:
011 44 7881 234 567
IDD (for US) +
UK COUNTRY CODE 44 +
MOBILE NUMBER (less the leading zero)
TOP TIP: If you travel a lot, then you can edit all of your phone numbers to always dial the full international number wherever you are in the world.
I have converted my entire phone book so that when I need to call someone I get straight through, and do not need to remember any international direct dialling codes, or country codes wherever I am in the world.
In the UK mobile example above, 07881 234 567 would simply be stored as +44 7881 234 567 in my phonebook and this works anywhere in the world, even if you are in the UK and dialling a local number.