Members who have opted out of SMS Communication by replying 'STOP' can be manually opted back in via their Account Settings (Located in the User Settings widget).
However, replying STOP will opt them out of both Club OS as well as Twillio (The SMS service utilized by Club OS).
Checking opt-in will give Club OS the go-ahead to try and send the message again, but delivery will fail, as they are still opted out of Twillio's system.
The Member would first have to reply 'START' to the message. This, and only this will opt the Member back in. Please note that if they have deleted that conversation thread from their phone, they can simply send 'START' to the designated SMS number for your location. Don't know the number off-hand? Just contact our Customer Success Team by emailing email@example.com for assistance!
Click here for further documentation regarding Twillio's protocol.