When texts are available
Text messaging has to be enabled for your account before anything can be sent by text. Settings → Notifications tells you the current state: available, restricted to numbers you have verified, or not set up. Email notifications work either way.
What can be sent by text
Booking confirmations, reminders and change notices, when text messaging is available, the customer has a mobile number, and you have turned the text channel on for that message.
Consent and opting out
Customers agree to receive texts while booking, and messages explain how to stop. Someone who replies STOP is opted out and continues to receive email instead.
Verification codes
Codes for confirming your own phone number are sent by text and are not affected by these settings.
Usage
Texts come out of your plan's text allowance, shown under Settings → Notifications → SMS Usage.
Keep them short
The important details live in the email; the text is the nudge.