Short codes are provisioned on a country-by-country basis. A short code is only able to send messages to, and receive messages from, same-country phone numbers on carrier networks that have approved the short code.
US short codes can only send to US phone numbers on US carrier networks that have approved that short code. If the end user is on a supported US carrier network but is out of the country (or in a US territory such as Puerto Rico or the US Virgin Islands), the message may be delivered successfully, but delivery is not guaranteed.
For Canada and UK short codes, the same principle applies. Please note that UK short codes’ coverage extends to England, Scotland, Northern Ireland, Wales, the Isle of Man and the Guernsey islands. UK short codes will not work for end users in the Republic of Ireland.
It is possible to send SMS messages worldwide from regular Twilio phone numbers using Global SMS.
For questions about which carriers Twilio short codes support, please see here.