Toll-Free SMS offers a great way to send Application-to-Person (A2P) SMS to recipients in the United States and Canada. However, there is a difference in how opt-out and opt-in (STOP and START) keywords are handled when using Toll-Free SMS.
Toll-Free SMS numbers have a layer of opt-out handling outside of Twilio that cannot be removed or customized. When an end user texts the word STOP to your Toll-Free number, the user will be opted-out of further messaging from your number, and will receive this auto-reply:
NETWORK MSG: You replied with the word "stop" which blocks all texts sent from this number. Text back "unstop" to receive messages again.
If the end user then sends the word START or UNSTOP, they will be opted back in to receive messaging from your number, and will receive this auto-reply:
NETWORK MSG: You have replied "unstop" and will begin receiving messages again from this number.
Notice: You will now see "You have replied "Start" ...." for START messages, and "Unstop" for UNSTOP messages. The keyword in the auto response will now match the word send in the original opt-in SMS.
Notice: Keywords such as "Help" and "Info" are not handled the same way. If the recipient is not opted-out of SMS from your number, and sends one of those help keywords, the Twilio default "Help" language (or Advanced Opt-Out custom "Help" language you've set) will be sent to the recipient. However, if they are opted out prior to sending a help keyword, they will not receive any auto-reply unless they opt back in first.
Learn more here Handle Opt-Ins with Advanced Opt-Out: Special behavior for Toll-Free numbers