What happens when a user sends stop to my toll free number?

The Opt-out flow for US toll free numbers is mandated by the US carriers and it is as follows:

A user sends the word 'STOP' to the toll free number.
The carrier sends the following outbound message to the user: 'NETWORK MSG: You replied with the word "stop" which blocks all texts sent from this number. Text back "unstop" to receive messages again.'

Any subsequent MT messages sent from the toll free number will fail with error code 9 - 'Illegal Number - you tried to send a message to a blacklisted phone number. That is, the user has already sent a STOP opt-out message and no longer wishes to receive messages from you.'

Can a user re-subscribe once he/she sent stop?

If the customer wishes to re-subscribe to the service, he/she can send a new message to the Toll Free number with the word 'UNSTOP'.
User 'STOP' and 'UNSTOP' requests are forwarded back to customers.
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