Hello @GeorgeLawrenceB ,
SMS is shorthand for the term "Short Message Service," or more commonly known as just texting. An SMS service is one that makes it so you can more easily setup mass-texting, automated responses, and other key elements for this communication, marketing, and updating channel. SMS Marketing
See also:
Getting started with SMS (PDF download)
Registering your account for SMS
SMS marketing overview
SMS marketing best practices
SMS marketing while respecting quiet hours
Enabling SMS for landing page sign up forms
Importing SMS contacts list
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