SIPSTACK Support for Opt-Out Keywords

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SIPSTACK Support for Opt-Out Keywords

When a recipient replies to a message from a SIPSTACK long code or toll-free phone number with any of the following keywords, they are automatically opted out from receiving further messages from that number:

STOP, STOPALL, UNSUBSCRIBE, CANCEL, END, QUIT

How Opt-Out Works

When SIPSTACK receives any of the above keywords as a reply:

1. A block list entry is created in SIPSTACK’s database for that recipient and number combination.
2. The message is passed to your configured webhook URL as an inbound message.
3. Any future attempt to send a message to that recipient from the same SIPSTACK number will return an HTTP 400 response.

Recipients can re-subscribe at any time by replying with an opt-in keyword. Once opted back in, messages can resume.

Important Behavior Details

• Single-word only: Only exact single-word replies trigger the block. “STOP PLEASE” or “PLEASE CANCEL” will not opt the user out — only “STOP” or “CANCEL” alone.
• Per-number block: The block applies to the specific SIPSTACK number that most recently messaged the recipient, not to your entire account.
• Applies even without a webhook: The STOP keyword is logged and the block is applied even if your phone number has no messaging webhook configured.

Caution

If you use multiple toll-free or long code numbers to message the same recipients, a STOP reply to one number does not automatically block the others. It is your responsibility to propagate opt-outs across all numbers in your program. SIPSTACK recommends maintaining a universal block list for your account.

Opt-In Keywords

Recipients can resume messaging by replying with any of the standard opt-in keywords. The specific keywords accepted depend on your configuration. Common opt-in keywords include: START, UNSTOP, YES.

Legal Notice

Text message campaigns may be subject to legal compliance requirements depending on the nature of your campaign and the jurisdiction of your recipients. While SIPSTACK handles opt-out keywords as described above, you should consult legal counsel to ensure your messaging program meets all applicable requirements in your operating regions.

Related Resources

• Opt-Out Keyword Handling for Toll-Free SMS
• Handling Deactivated Phone Numbers
• What Kind of Messages Are Restricted on SIPSTACK SMS?
• Contact SIPSTACK Support