USA phone numbers follow a standard 10-digit format (NPA-NXX-XXXX) under the North American Numbering Plan, consisting of a 3-digit area code, a 3-digit central office code, and a 4-digit line number. They are often written as (XXX) XXX-XXXX or +1-XXX-XXX-XXXX for international calls, with "+1" as the country code.