Calling Saint Helena from the United States explained:

• 011 - US exit code; dial first for international calls made from the USA or Canada
• 290 - Country Code for St. Helena (includes Tristan da Cunha Island)
• Phone Number - 5 digits (except for Tristan da Cunha which still uses 4 digit phone numbers)
• US to St. Helena international dialing format: 011 290 ?? ???

Starting digits for main areas:

• Jamestown: 2
• Tristan da Cunha: 8

Phone Books:

• Sure Group Directory - official phone book of St. Helena island

Updates:

• Saint Helena telephone numbering changes - October 2013