STOP is the universal way clients can opt out of marketing texts. It's required by carrier rules, and it's there to protect them from spam.
What STOP does
When a client texts STOP to a message from your business, they stop receiving marketing texts from you — like reminders and win-back messages. They will still get the booking confirmation if they make a new appointment, because that one is a transaction, not marketing.
How to see who opted out
Open Clients and look for the small "Opted out" tag on their profile. You can also filter by it from the filter sidebar.
Getting a client back on the list
Only the client can re-subscribe — by texting START to the same number they opted out from. We can't do it for them, and neither can you. This is a legal requirement, not a Callisto thing.