> This page is part of the [Customer.io documentation](https://docs.customer.io). For the complete index, see [llms.txt](https://docs.customer.io/llms.txt).
> Last updated: July 10, 2026

# Opt-in and out flow

You need to provide your users with clear instructions to opt into and out of your SMS messages. You must have designs for your opt-in and out flow before you request a sender number. Twilio and carriers evaluate your opt-in and out flow as part of your registration process.

## You must preview your opt-in and out flow before you register as an SMS sender

You do not need to have completed your opt-in and out flow before you contact Customer.io to register as an SMS sender—in part because you won’t be able to do it until you have a sender number!

But you need to have designs or a preview to demonstrate your opt-in and out flow. Twilio and carriers evaluate your designs to make sure that you’re ready to send messages.

## What format should my opt-in and opt-out preview be?

It’s a best practice to have your opt-in and out flow in a live, preview-able state. It doesn’t need to be publicly accessible; you might place your opt-in flow on a staging website or otherwise hide it from your public site. But providing a functional URL makes it easier for Twilio and cellular carriers to preview and test your opt-in flow.

If that won’t work for you, a PDF design of your opt-in and out flow is sometimes an acceptable alternative.

## Writing opt-in and opt-out instructions

We partner with Twilio to send SMS messages. By default, Twilio [supports a number of keywords](https://help.twilio.com/articles/223134027-Twilio-support-for-opt-out-keywords-SMS-STOP-filtering-) to opt into or out of messages. You may want to use their list of keywords in the opt-in and opt-out instructions that you add to your [privacy policy](#update-your-privacy-policy).

Beyond your privacy policy, make sure you gather explicit consent for marketing messages, even before you request a sender phone number. This is typically a checkbox on your signup form that lets users opt into marketing messages.

For the specific elements your consent surface must include, plus an example form, see [Build a compliant SMS opt-in form](/messaging/channels/sms/registration/sms-opt-in-form/).

*   *   [You must preview your opt-in and out flow before you register as an SMS sender](#you-must-preview-your-opt-in-and-out-flow-before-you-register-as-an-sms-sender)
    *   [What format should my opt-in and opt-out preview be?](#what-format-should-my-opt-in-and-opt-out-preview-be)
    *   [Writing opt-in and opt-out instructions](#writing-opt-in-and-opt-out-instructions)

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