Opt-In compliance 

If the compliance is Opt-In, you can only reach out to your recipients via SMS if you received their consent for sending text messages and if the Opt-In field in that record is true.

Opt-out compliance 

By default, we provide the SMS Opt – out field on Lead and contact object. You can add the opt-out disclaimer message at the end of the message body and setup up an automation process which would automatically check the SMS Opt-out field present on the record whenever there is a reply with opt-out keyword.

We also have Opt-out functionality on SMS-Magic portal where you can configure the opt-out keyword.