The system does not automatically include an unsubscribe link or text in a SMS.

The Opt Out / Unsubscribe process will depend on whether you send from an actual mobile number as your sender ID or use an alpha numeric sender ID ( Which can not be replied to directly) on how the process should be addressed.

When using a mobile number Include "Reply Stop to opt out" or if using an Alpha Numeric ID ( ie MyAgency ) include Reply Stop to [your mobile number] to opt out. whoever receives the SMS will need to manually unsubscribe the contact, you may want to clarify if they want to be unsubscribed from SMS or All Communications.