This API only works if the lead has unsubscribed via Apten”s smart-unsubscribe feature, or via user-defined custom unsubscribe keywords. Leads who opt-out via a default keyword (e.g. “STOP”, “UNSUBSCRIBE”, “CANCEL”, “END”, “QUIT”, and “STOP ALL”) are ineligible for resubscription.This API also turns Apten on for the lead.