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.
"You have successfully been unsubscribed. You will not receive any more messages from this number. Reply START to resubscribe."
When that contact resubscribes, they'll receive the following message:
"You have successfully been resubscribed to messages from this number. Reply HELP for help. Reply STOP to unsubscribe. Msg&Data Rates May Apply."
For more information on proper SMS or IVR-based communication, consult our TCPA compliance guide.