Calling someone in the United States on his mobile is no different than calling him on his land line. If a caller is outside of the United States and wants to call an American mobile phone, there is a specific format of numbers that must first be dialled. It should also be noted that international calls are more expensive than local calls. The extra charges are usually applied to the caller, and can be quite high.
Find a resource for locating the appropriate country code to dial. For many countries, calling the United States requires the use of the country code "00". However, for some originating countries, it is different. Use a search engine to look up "international call to U.S. codes". Click on one of the websites on the results page.
Determine the correct country code to use by inputting the originating country into the correct field on the chosen website. Select the destination country as "United States". Click "Submit" to run the query.
Use the resulting data to make the call to the American mobile phone. The results page will show a country code to use and it will also show the need to add a "1" after the country code. Use the mobile number you have, including the area code, and dial the string of numbers as: country code-1-area code-number. If the area code is unknown, the next page on the website will provide the resource for looking that up by state.