The business you are signed up to must request to enable this feature before you can receive SMS notifications.
To receive notifications via SMS for waitlist announcements (e.g. if a space opens up for you) you will need to add your phone number on your notifications page. Head to My Account>>Account Settings>>Notifications
Select the check box to receive SMS messages for Waitlist announcements. Then enter your phone number and request a verification code.
You'll then be prompted to add your verification. Once you've done that you'll start receiving SMS notifications for waitlist announcements.
Tip. Don't forget to choose the correct country from the drop-down.