When dialing from outside the US, the US's country code (1) must be used before the phone number. The country code sometimes is surrounded by parenthesis or has an addition sign (+) before it.
US phone numbers start with a 3 digit area code. If there is a country code, there is typically a space between it and the area code. The area code is typically surrounded by parenthesis.
After the area code is a 3 digit telephone prefix. There is typically a space before the telephone prefix to differentiate it and the line number from the area code.
Following the telephone prefix, is a 4 digit line number. A space or dash is typically used to separate the telephone prefix from the line number