A Double Opt-in is an added layer of protection against accidental and unwanted opt-ins to a mobile campaign by requiring that contacts reply to a text message confirming their consent to join. 

The process begins when a contact attempts to join your mobile program (ex: when the contact texts your opt-in keyword to your messaging code). The contact will then receive a Double Opt-In message asking that they confirm their request to join your mobile campaign. By replying with YES, Y, OK, or SURE, the user confirms their opt-in request and is “Double Opted-in” to your mobile campaign. The user then receives your interactive 001 confirmation message that includes a required footer, "Reply STOP to cancel, msg&data rates may apply." 

See Related Article: Messaging > Opt-In Filter: Double Opt-In & Age Verification