Because an alphanumeric sender ID can only be used for one way messaging, the recipient will not be able to reply back to your messages. If the user no longer wants to receive your messages, they need a way to tell you that. If users attempt to opt-out, but they keep receiving messages because their opt-out request never reached you, they will often report your message as spam to their carrier. This can result in all of your messages being blocked by that carrier, often without letting you know.
It is important to ensure that customers receiving messages using an alphanumeric sender ID have opted in to your service and that you supply an alternative means for them to opt out. Consider including in your messages a web link, email address, or a different phone number that they can use to opt out.