Related Clues:

• Dialing prefix
• 315 or 413, phone-wise
• Three numbers before seven
• One of seven for New York City
• First digits dialed, often
• Triple digits?
• 307, for all of Wyoming
• Numerical trio
• Opening digits America's used with most of communication lines?
• Phone number part
• Parenthetical trio
• First three digits of a 10-digit phone number
• Digits in parentheses
• Cape Canaveral's 321, for one
• 213 or 323 for Los Angeles, e.g