If you manually Opt Out a Contact, they will receive a message saying:
"You have been manually unsubscribed. Please reply with 'START' to opt back in to receiving messages from this number"
If you wish to opt a Contact back in to texting, you can Request Opt In for Phone Number and they will receive this message:
"Please reply with 'START' to opt back in to receiving messages from this number"
And if a Contact opts themselves in by texting you 'START' they will receive an automated message saying:
"Reply STOP to unsubscribe. Msg & Data Rates May Apply."
If you have any further questions about this, please reach out to Support at firstname.lastname@example.org.