Opt-outs are common among SMS ("Robotext") and IVR ("Robocall") services. According to the TCPA (see our guide here), your contacts should have the ability to do so at any time. Use this guide to learn how to track and manage these contacts via Twilio and your TextIt account.
How Twilio Manages Opt-outs
When a contact sends STOP, STOPALL, UNSUBSCRIBE, CANCEL, END or QUIT to one of your Twilio numbers, Twilio will prevent them from receiving any additional messages until that contact responds START. Specifically, they'll receive the following message: