The system uses Twilio to send out SMS (text message) notifications. There will be times where your customers want to opt-out of the service. Twilio offers you the option to set specific keywords like "STOP" or "UNSUBSCRIBE" so that your customers can reply to the incoming text with that keyword if they want to be removed from the notifications list.

Once you set up the keyword you want to use, after a customer decides they no longer want to receive text message reminders, they will simply respond with your keyword of choice. 

You can also set up "START" as a keyword so that your customers can start receiving the SMS messages again.

If you follow the article below, it will explain how to configure the opt-in/out keywords.

https://support.twilio.com/hc/en-us/articles/223134027-Twilio-support-for-opt-out-keywords-SMS-STOP-filtering-

Note: If you are using the campaign feature and configured the same Twilio account as you use for your booking notifications, the customer will also stop receiving booking notifications if they opt-out from the SMS service. We recommend using a separate account for your campaigns to avoid the booking SMS notification issue.