Twilio, by default, handles standard English-language reply messages such as STOP, UNSTOP, UNSUBSCRIBE or CANCEL for Toll-Free and Long Code messages, in accordance with industry standards. Supported opt-out and opt-in keywords for Twilio Programmable Messaging are listed below.
STOP, STOPALL, UNSUBSCRIBE, CANCEL, END, or QUIT
Any of these replies will stop a customer from receiving new messages from that particular Twilio number. When attempting to send a message to a subscriber that previously responded with any of the keywords mentioned above, Twilio's API will throw a 400 response with Error Code 21610 - Message cannot be sent to the 'To' number because the customer has replied with STOP.
Please note that these replies will only apply to the most recent number that messaged the customer. If you are using more than one Toll-Free number or Long Code for sending messages, you must take additional action separate from Twilio to prevent these customers from receiving messages from your other numbers. This does not apply to Twilio Messaging Services, however. A STOP reply to a Messaging Service will block new messages to this number from all of the other numbers in the service.
In addition, only single-word messages will trigger the block. So, for example, replying STOP will stop the customer from receiving messages from that particular Twilio number, but replying STOP PLEASE will not. Further, the block works and is logged even if your number currently has no messaging request URL.
START, YES and UNSTOP
Any of these replies will opt customers back in to the messages coming from a Twilio Long Code. These messages will also be delivered to your Twilio account, and the defined web hook, so you can update your application logic.
Please note that Toll Free numbers only respond to UNSTOP.
HELP and INFO
Either of these replies will return a message informing the customer that they can use the above commands to control the delivery of messages. Twilio does not forward these messages to your web hook.
It is currently not possible to customize the default response triggered by these keywords.
STOP Filtering for Short Codes
Twilio does not handle default STOP filtering for short codes. For information on short code HELP and STOP responses, please see this article.
Text message campaigns may be subject to various legal compliance requirements depending on the nature of your text messaging campaign, the location from where you are sending your text messages, and the location of your recipients. While Twilio will handle the above-described messages on Long Codes and Toll-Free numbers in the way described in this article, you should consult with your legal counsel to ensure that your text messaging campaign conforms will need to meet all applicable legal compliance requirements.
If you have further questions on this, please contact Twilio Support.