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SMS is a regulated channel that requires specific functionality for managing subscriptions. OneSignal provides default consent keywords for common actions like subscribing, unsubscribing, and requesting help. You can also create custom keywords. Manage consent keywords by navigating to Settings > Platforms > SMS Settings > Consent Management.
OneSignal SMS consent keyword management settings showing opt-in, opt-out, resubscribe, and help keyword configuration
Each SMS Sender must be configured to sync incoming message replies for keywords to work. Go to SMS Settings > Senders and click Setup Replies.If message replies are not synced, OneSignal still stops sending to opted-out recipients, but you cannot update their subscription status.Alpha-numeric sender IDs (e.g., “MyBrand”) do not support two-way messaging and cannot receive replies. If you use alpha-numeric senders, you must implement your own opt-out mechanism (such as a web link or dedicated phone number) and manually manage subscription preferences to meet local regulations.

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Double opt-in

Double opt-in defaults SMS subscriptions to a pending state when added to OneSignal and automatically sends a prompt for the user to confirm. Once the user replies with the opt-in keyword, OneSignal marks the SMS subscription as subscribed. Double opt-in ensures genuine user consent, reduces spam complaints, and helps comply with regulations like TCPA and GDPR. See SMS Regulatory Compliance for details. To enable double opt-in:
  1. Click Edit next to Double Opt-In.
  2. Toggle on Send Message Prompt.
  3. Select or create the SMS template to use as the opt-in prompt.
OneSignal double opt-in settings showing the message prompt toggle and template selection
Example prompt: 
Hi {{first_name | default: "there"}}, to subscribe to OneSignal SMS, reply SUBSCRIBE to start receiving messages.
This example uses Message Personalization to include the user’s name if collected via tags, defaulting to “there”. The keyword SUBSCRIBE tells the user what to reply to opt in.
Keep your prompt short to minimize SMS costs. See SMS pricing for details.

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Text-to-subscribe keywords

Create opt-in keywords that match what you used in the message prompt. After the user replies with the keyword, you can send a confirmation message letting them know how to unsubscribe.
OneSignal text-to-subscribe keyword settings with keyword input and reply template
Example confirmation: 
Thanks for subscribing to SMS messages! We will only send you important information. Reply STOP to opt out at any time.

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Opt-out keywords

Opt-out keywords like STOP, UNSUBSCRIBE, and CANCEL let users unsubscribe from messages.
STOP, HELP, and START are protected compliance keywords that cannot be changed via the OneSignal dashboard or API. To change their response messages, contact support@onesignal.com with your App ID and the new response message.
OneSignal opt-out keyword configuration showing default keywords and sync settings

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Sync unsubscribe across all senders

This setting controls how opt-outs are applied:
  • On: When a user texts STOP or another opt-out keyword, their subscription status updates to unsubscribed on their profile. No sender can message them.
  • Off: The opt-out only prevents messages from the specific sender number. Messages from other sender numbers may still go through. This is recommended if you use separate numbers for marketing and transactional messages.

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Resubscribe keywords

Resubscribe keywords like START, UNSTOP, and YES let users opt back into messages after opting out.
START is a protected compliance keyword that cannot be changed via the OneSignal dashboard or API. To change the response message, contact support@onesignal.com with your App ID and the new response message.
OneSignal resubscribe keyword configuration showing default keywords and reply template
Once a user is added to a sender’s suppression list, there is no way to remove them through the OneSignal dashboard. The user must text a resubscribe keyword to the same sender number. This updates their subscription status to subscribed and removes them from that sender’s suppression list.

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Help keywords

Help keywords like HELP provide users with information about how to manage their SMS subscription.
HELP is a protected compliance keyword that cannot be changed via the OneSignal dashboard or API. To change the response message, contact support@onesignal.com with your App ID and the new response message.
OneSignal help keyword settings showing the default HELP keyword and response template

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FAQ

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Can I change the STOP, HELP, or START response messages?

Not through the dashboard or API. Contact OneSignal Support to update the response for these protected compliance keywords. No. Alpha-numeric sender IDs cannot receive replies. If you use alpha-numeric senders, implement your own opt-out mechanism (such as a web link or dedicated phone number) and manage subscription preferences manually.

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What happens if message reply syncing is not configured?

OneSignal still prevents sending to opted-out recipients, but the subscription status on the user’s profile is not updated. Configure reply syncing under SMS Settings > Senders > Setup Replies.

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How does “Sync unsubscribe across all senders” work?

When enabled, an opt-out keyword from any sender number updates the user’s subscription status to unsubscribed globally. When disabled, the opt-out only suppresses messages from the specific sender the user replied to.

SMS keywords

Create custom keywords for engagement, tagging, and segmentation.

SMS regulatory compliance

Requirements for SMS consent, opt-out, and messaging regulations.

SMS setup

Configure SMS senders, reply syncing, and platform settings.

Message personalization

Use Liquid tags and data to personalize SMS content.