Contacts can unsubscribe from SMS messages by replying “STOP” to any of the SMS messages you send to them. Learn more about SMS STOP clause requirements.
Once a contact unsubscribes from your SMS messages, they will skip over any “Send SMS” actions they encounter in your automations. This action will not unsubscribe them from your email lists and there is no way to track which contacts have unsubscribed from your SMS messages.
In addition, it is not possible to manually unsubscribe contacts from receiving your SMS messages. However, you can remove their phone number from the default phone number field to prevent them from receiving SMS messages from you in the future.
If a contact wishes to re-subscribe to your SMS messages, they can reply to one of your SMS messages with START, YES, or UNSTOP. It is not possible for you to manually re-subscribe them.