+1 belongs to 20 places, not just the United States
NANPA, the organization that assigns every area code used across the US, Canada, and the Caribbean, counts 20 member countries and territories that all dial out under the shared +1 country code: the United States and its territories, Canada, and 18 Caribbean nations and territories, from Anguilla to Turks and Caicos. Nothing about a +1 number's format signals which of those 20 it actually belongs to. The country code by itself only narrows a call down to North America and the Caribbean, not to a specific country. The three-digit area code that follows +1 is what actually does that work, the same way it does inside the US (see how US area codes actually work).
NANP members outside the US and Canada, and their area code(s)
| Country/territory | Area code(s) |
|---|---|
| Anguilla | 264 |
| Antigua and Barbuda | 268 |
| Bahamas | 242 |
| Barbados | 246 |
| Bermuda | 441 |
| British Virgin Islands | 284 |
| Cayman Islands | 345 |
| Dominica | 767 |
| Dominican Republic | 809, 829, 849 |
| Grenada | 473 |
| Jamaica | 876 |
| Montserrat | 664 |
| Sint Maarten | 721 |
| St. Kitts and Nevis | 869 |
| St. Lucia | 758 |
| St. Vincent and the Grenadines | 784 |
| Trinidad and Tobago | 868 |
| Turks and Caicos | 649 |
Most of that list didn't exist as separate area codes until the 1990s
Until the mid-1990s, nearly all of the NANP's Caribbean members shared a single area code, 809, with no way to tell one island's number from another's just by looking at it. NANPA split that code apart in stages: Bermuda's 441 went first, in 1995, followed by Puerto Rico's 787 on March 1, 1996, Barbados' 246 on July 1, 1996, and the Bahamas' 242 later that same year, on October 1. Saint Lucia's 758 began permissive dialing in July 1996 as well, Saint Kitts and Nevis's 869 that October, and Saint Vincent and the Grenadines didn't get its own 784 until June 1998. The Dominican Republic split off from 809 over an even longer stretch, adding overlay codes 829 and then 849 as the original code ran out of numbers to assign.
Sint Maarten's 721 is the newest addition on the list: it wasn't assigned until 2011, the year after Sint Maarten became its own constituent country within the Kingdom of the Netherlands rather than a district of the old Netherlands Antilles. The numbering plan keeps tracking political geography, not just running out of digits.
How to actually dial a US number from abroad
Calling a US number from outside the NANP works the same way as calling any other +1 destination: dial your own country's international exit code, then 1, then the 10-digit US number (area code plus the 7-digit line). Most of the world dials out with 00 as that exit code; the US and Canada are two of the few places that use 011 instead, which only matters if you're calling out of North America, not into it. On a mobile phone or most VoIP apps, a leading + does the same job as the exit code and works from almost anywhere.
A US number written as (415) 555-0142 becomes 00 1 415 555 0142 when dialed from the UK or most of the rest of the world, or +1 415 555 0142 from a mobile.
A +1 number isn't automatically American, and it isn't automatically safe either
The shared +1 code has a real practical downside: eight of the Caribbean NANP codes above (268, 284, 473, 649, 664, 767, 809/829/849, and 876) are the specific area codes tied to the "one ring" scam, where an autodialer calls once and hangs up, hoping the recipient calls back into a premium-rate international line. See Is There Really Such a Thing as a 'Scam Area Code'? for how that scam works and what the FCC and FTC recommend doing about it.