Understand SMS filtering by wireless carriers

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Learn reasons why wireless carriers filter SMS messages and best practices for avoiding a failed delivery. 

As with email providers (Google, Yahoo, etc.), mobile carriers filter messages that they think are spam. However, unlike with email, wireless carriers will sometimes not deliver your messages. If this happens, you may see that the SMS failed delivery due to a “Carrier Violation” error.

Reasons for SMS filtering

Mobile carriers filter SMS messages for two main reasons:

  • Protect their customers from receiving spam messages
  • Meet regulations for sending SMS to phones in a specific country or on a certain mobile network

The way SMS messaging is filtered varies country by country and between different mobile networks. 

Further, carriers will filter messages that contain terms related to sex, hate, alcohol, firearms, and tobacco (SHAFT). SHAFT words are prohibited by the CTIA, the regulatory body which enforces guidelines for SMS messages. Further, terms that are considered or relate to profanity, controlled substances, or CBD are similarly filtered.

Receiving an SMS error message

Wireless carriers do not always report when they filter an SMS message, as they don’t want spammers to learn how to get around their filters. If they do report that a message was filtered, this information will show in Klaviyo as a Failed Delivery with the Failed Reason being Carrier Violation. Note that they won't always give a reason either, and you will sometimes see the Failed Reason as Unknown

To view this information for a text message:

  1. Navigate to the SMS flow or campaign you want to look at 
    1. For flows: click the SMS message and then click View all Analytics on the left-hand side
    2. For campaigns: go directly to step 2
  2. Select Recipient Activity
  3. Select the Other dropdown menu
  4. Choose Failed Delivery

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What to test when you receive a failed delivery error

If you encounter this error, we recommend running tests to see how much you're being filtered. Test this by: 

  • Using phones on different wireless networks
  • Sending your message from a different number/having multiple numbers on your account to send messages from
  • Using different content in the message body 
  • Sending without a URL shortener, as carriers often view third-party shorteners (e.g., bit.ly) as suspicious 

Best practices for not being filtered

To try to prevent being filtered by mobile carriers, it’s important to follow best practices for SMS messaging, including:

  • Always provide a way for subscribers to opt out; carriers heavily filter messages that don't include STOP language
  • Provide value to subscribers
  • Have double opt-in enabled for SMS lists
  • Let subscribers know who is sending the message
  • Do not send SMS messages early in the morning or late at night, as a text message from a business during off-hours may be seen as intrusive
  • Do not overload SMS subscribers by limiting texts for each recipient to two to four SMS/MMS per month
  • Use a personal tone to sound more human
  • Avoid SHAFT-related words
  • Use a trusted sending number, such as a short code or verified toll-free number 

Additional resources

Learn more about SMS messaging by reading the following Help Center articles:

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