If you’re sending an SMS to recipients and your Sender ID is an Alphanumeric Sender ID (custom word), it’s important to offer clear opt-out rules if required, as two-way messaging isn't supported (i.e., Recipients will not be able to reply to your SMS sent from an alphanumeric Sender ID).
If recipients no longer want to receive your messages, we recommend that you provide your users with different opt-out options such as:
Sending an email or making a phone call to your support team
Texting another long code or short code to opt-out (i.e.: a Virtual Number)
Using a URL in your message to offer an opt-out option