This feature is very similar to your DNC list. Once the lead replies with the selection of Opt Out, the system will add the lead to the SMS Opt Out List. Leads in the SMS Opt Out list will no longer be dialed.  Functionality is very similar to the DNC functionality.  

To add a number to the Opt Out List, click Upload SMS Opt-Out.  To export your SMS Opt-Out list, click Export SMS Opt-Out.  To delete results, click the Delete Results button (select a single number, multiple numbers or all numbers). 

Note:  Be very careful when deleting results as this action cannot be reversed.