A valid U.S. phone number typically consists of ten digits, with the first three digits being the area code, followed by a three-digit prefix, and ending with a four-digit line number. The area code cannot start with a 0 or 1, and the second digit of the area code cannot be a 9. Additionally, the prefix cannot start with a 0 or 1. Valid U.S. phone numbers may also include country codes for international calls.