Indicates whether the customer has opted in to receive SMS communications. Customers must explicitly opt-in (verbally to a sales rep or digitally) before any SMS messages can be sent. This is required to maintain compliance with FCC, TCPA, and CTIA regulations. If not provided, the default value is false and no messages will be delivered. This value can be updated via the API or UI unless the customer has explicitly replied "STOP" to an SMS. In that case, they cannot be opted in unless they text "START".