> This page is part of the [Customer.io documentation](https://docs-customerio.netlify.app). For the complete index, see [llms.txt](https://docs-customerio.netlify.app/llms.txt).
> Last updated: August 22, 2025

# 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-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?[](#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[](#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, you should make sure that you already gather explicit consent for marketing messages—even before you request a sender phone number. This is often something like a checkbox on your signup form that allows users to explicitly opt-in to receive marketing messages—assuming you send marketing messages.

### Example opt-in and out flow[](#example-opt-in-and-out-flow)

For example, you might include the following with a checkbox on your site:

Explicit Opt-in Example

By entering your phone number and checking the box, you agree to receive marketing messages from <Business name> at the phone number provided above. You may receive up to 4 messages per month. Data & message rates may apply. Reply STOP to opt out and HELP for help. \[Privacy Policy Link\] \[Terms of Service Link\]