When adding/editing a patron there is a hint next to the SMS number field: "Please enter numbers only. Prefix the number with + or 00 if including the country code." That ins't always necessarily true. This should be an HTML customization.
Can you give an example on when this isn't true?
Twilio is a messaging provider that many libraries use. They require that the SMS number be in the following format: +11234567890. The Koha instructions are not necessarily incorrect, but they are a bit misleading. Currently we have to use jQuery if we want to clarify the instructions, such as this: $('#pat_memberentrygen .sms_number_help').text("Enter +1 followed directly by the ten-digit number (+11234567890)");
In a more general sense, since Koha's native Email to SMS is deprecated, libraries are moving to using third-party providers that sometimes have different requirements for number formatting.