If you would like to send out an SMS 3 days after someone subscribed, here is the hook that you have to use
add_action('wp_sms_add_subscriber', 'sendWelcomeSmsToNewSubscribersAfter3Days', 10, 2);
/**
* @param $name
* @param $mobile
*/
function sendWelcomeSmsToNewSubscribersAfter3Days($name, $mobile)
{
WPSms()->scheduled()::add(
date('Y-m-d H:i:s', strtotime('+3 day')),
WP_SMS\Option::getOption('gateway_sender_id'),
sprintf('Hello %s', $name),
array($mobile)
);
}