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.