Why are my SMS messages not being delivered to Sprint or T-Mobile customers?

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Why are my SMS messages not being delivered to Sprint or T-Mobile customers?

Sprint and T-Mobile are both known to block promotional SMS messages from being delivered to their customers’ phones. This is an industry-wide problem with SMS. Although the messages are not going through, they will still show as delivered in Klaviyo, as we do not have insight into these blocks occurring.

To avoid these carriers blocking your SMS messages to your customers, you can try the following:

  • Make your messages seem more “human”. Use conversational language that will not trigger the blocks.
  • Avoid using many links in your messages.
  • If your customer’s carrier is Sprint, they can text “allow phone number” (ex: allow 5555551000) to the number 9999.
  • If your customer’s carrier is T-Mobile, they can call T-Mobile Support (1 (800) 937-8997 in the US), or they can chat live with support to have your number unblocked.


Klaviyo is looking into a long-term resolution for this issue, but in the meantime, these solutions should allow your Sprint and T-Mobile customers to receive SMS messages from your Klaviyo account.

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