For non-North American recipients from a North American phone number, you have to prefix with the country code[0]. So unnecessary if you're in the US and want to call Canada or Jamaica or wherever. But say you want to call someone with a UK phone number, you just send the SMS to +44-<their phone number>.
As a leftover artifact from growing up not allowed to call long-distance, I frequently think of things like "Is long-distance texting charges a thing? Will I get charged for texting my friend with a Mexico number? Does it matter if she's currently in Mexico or the US? Does sending a picture incur a fee even if an sms doesn't?"
[0]: https://en.wikipedia.org/wiki/List_of_country_calling_codes