Four pieces, always in the same order.
The exit code tells your phone company that the next number is in another country. From the United States, Canada and the Caribbean it is 011. From the United Kingdom and most of Europe it is 00. A few countries have more than one. We show the usual one and list the others.
The country code is the same no matter where you call from. The UK is 44, Mexico is 52, Singapore is 65. The US, Canada and many Caribbean countries share 1. On a call inside +1, you dial 1 and the area code. You do not dial 011.
Inside a country, people often dial a 0 before the city code (020 for London, 01 for some other places). When you call from abroad, drop that 0. Italy is the exception people trip over: the 0 is part of the number, so Rome stays 06. Mexico no longer uses the old 01 trunk. Do not dial it.
A landline usually has a city code and then a local number. A mobile usually does not: you dial the exit code, the country code, and the mobile number. Argentina is different. For an Argentine mobile, dial 54, then 9, then the area code, and drop the 0 and the 15 that are used inside Argentina.
Hold 0 until you get +, then dial the country code and the number. The + works from any country, so you can skip the exit code. Example: +44 20 7946 0123 for a London landline.