We need to be able to send an opt out confirmation SMS right when they opt out that will let them know they will no longer receive messages from us but they can reply START/SUBSCRIBE to opt back in anytime.

Right now, because they opt out, we can no longer text them until they opt back in. Having at least a final automated SMS that confirms they have successfully opted out and instructing them how to opt back in would be extremely beneficial for keeping the conversation going if the client so chooses. That way if they did it by accident they can quickly come back in