The text messaging gateway here doesn't actually use SMS; it relies on the fact that many carriers have an email-to-SMS gateway of *some* sort. (That's why carriers have to be set up on DW individually, and why they don't all work and sometimes stop working; if the carrier doesn't have an email-to-SMS gateway or their message format changes, it will stop working.)

Sending things as actual SMS messages would bypass all of that, but be immensely more complex. And more expensive.