This is required by law and there is no way to change this process.
In some cases a customer may use the word "stop" in a reply like "please stop the work" and the system accidentally opts them out from receiving text messages from you.
In order to have the customer opt back in to receive your text messages, simply have them reply to your text message, or text 313131 the message "textoptin".
If this works, they will receive a text back confirming their subscription.