If your customer or client replies STOP to a text message, then they'll be unsubscribed from receiving future texts. This means that appointment reminders and message blasts will be blocked.
If someone has opted out of reminders but wants to resubscribe, they can simply text "START" or "YES" to the same number they used to opt out. You can find the phone number used for sending reminders and blasts from your account in Settings > Account. Only verified dedicated phone numbers will be displayed. No phone numbers will be shown during the trial period, and a blast number will be shown if it has been set up.