If your contacts send a single-word unsubscribe command to your Textline, they will no longer receive any SMS messages from you. Unsubscribe / opt out commands are STOP, STOPALL, UNSUBSCRIBE, CANCEL, END, and QUIT. A contact who has blocked your texts can resume receiving messages from your Textline by sending a single-word SMS message with the command START or YES.
If a customer unsubscribes, you will see the error message Customer Unreachable by SMS, and the message box within their Conversation History will be greyed out. Any other messages (Announcements, etc) that you may attempt to send to them will not go through.
The customer will receive the final message, "You have successfully been unsubscribed. You will not receive any more messages from this number. Reply START to resubscribe."