How to call a phone number in London from the United States
The number you have for London might be presented in a number of different ways:
+44 (0)20 5555 5555 (this has the UK country code)
020 5555 5555 (this assumes you're calling from within the UK)
5555 5555 (very unlikely, because it doesn't include the area code)
In each case you'll need the country code (44 for UK), and the area code, minus the leading zero (20 in the case of the example above).
To call any international number from the US, start with 011 first (from a landline phone) or hold down the 0 button until it turns into a + (from a cellphone).
011 44 20 5555 5555 (from a regular landline phone)
+44 20 5555 5555 (from your cellphone)