Because of the number of mobile and land line phones there now are in the world, numbers are often repeatedly used in different countries. To tell your provider that you wish to phone outside of the country you have to enter a specific code. Once you have done that you need the country's area code to tell them which country you are phoning. This may sound confusing and complicated but most codes can be found either in your phone's settings, or in a standard diary.
Find the IDD prefix. This is the international dialling code that lets the provider know you wish to phone outside the country you are currently in. In the USA it is 011.
Look up the country code you are dialling to. This will usually be next to the IDD prefix.
Input the "+" key on your phone, then dial the IDD prefix followed by the country code. Insert the rest of the mobile number after the codes and then press dial. Your call should connect.
The process is the same regardless if you are phoning from a mobile or a landline.
Some mobile phones require to be activated to work abroad, otherwise they cannot receive or send calls.