By default, Twilio handles opt-out and opt-in keywords as described in Twilio support for opt-out keywords (SMS STOP filtering) document; when Twilio receives an opt-out keyword from an external sender phone number to any Twilio number, Twilio will reply with a pre-generated response and will no longer send messages from the Twilio Number to that external phone number until the sender opts back in by sending an opt-in keyword.
When using SMS with Proxy, there are special considerations and deviations from the standard handling of opt-out and opt-in keywords:
1. Proxy will not forward STOP, UNSUBSCRIBE, END, CANCEL, QUIT, STOPALL, ARRETT, ARRET, ARRETE (opt-out) messages from either participant. Dropped messages do not trigger callbacks.
2. Proxy forwards all opt-in, help and info keywords between Participants, except UNSTOP, to ensure that potentially valid messages are not lost.
3. For any opt-in/out message not forwarded, Proxy will send a debugger notification with Twilio code 80901.
4. Proxy does not support the Advanced Opt Out feature available with Messaging Services.
5. Unlike with a Messaging Service, opt-out keyword replies will not apply to the entire pool of phone numbers in a Proxy Service pool.
6. If you would like Proxy to respect the STOP SMS Filtering flag, you may submit a request to firstname.lastname@example.org to enable the ProxyRespectSmsStopFilterFlag account flag. Once Twilio enables this, custom opt-in/out keyword handling can be accomplished by adding the desired logic to Proxy's InterceptCallbackUrl.