SMS Carrier Filtering in the United States and Canada

This guide discusses SMS filtering by wireless carriers in the United States and Canada that can affect delivery of your Twilio Programmable SMS messages. In this guide, you will learn how and why messages are filtered, and options for improving your delivery rates. For a general overview of wireless carrier filtering in all locations Twilio services, please see How Does Carrier Filtering Work. For U.S. and Canada specific carrier filtering information, continue reading.

Carrier Filtering Overview

Most carrier filtering in the U.S. and Canada takes place on SMS traffic sent using standard 10-digit long code phone numbers. According to U.S. and Canadian carrier regulations, Application-to-Person (A2P) traffic should not be sent via local long code numbers; this traffic should instead be sent using short code numbers (or Toll-Free numbers for some types of business messaging).

A2P traffic is how the industry refers to messages like automated alerts, notifications, one-time password (OTP) login codes, or any other kind of traffic originating from an application. For more information, please see What is P2P and A2P Messaging.

Recent Developments

As of 2019, carriers’ rules about long code A2P messaging are beginning to change. A major example is Verizon's upcoming 10 digit long code (10DLC) A2P service. This is a service that will specifically allow A2P traffic to be sent using local numbers to Verizon subscribers, with the added cost of a small delivery fee. For full details, please see Commercial Long Code SMS Product and Fee Structure Changes on Verizon.

As of this writing, Verizon is the only carrier in the U.S. and Canada that has announced a solution for this. However, Twilio expects that other carriers will come out with similar solutions in the future.

Filtering Mechanisms

Carriers in the U.S. and Canada appear to be using adaptive (machine learning) software systems to protect their users. These systems look at both message content and volume, and behave very much like email filtering systems. Messages receive a cumulative score based on how many messages have come from a phone number during a certain time period, how many similar messages have transited the carrier’s network, or if the message contains content that makes it a high match for spam or A2P. Time periods are measured by the second, minute, hour and day.

Carriers in the U.S. and Canada usually report to Twilio when a message has been filtered. If you suddenly see that a large number of your messages are resulting in a status of Undelivered with a 30007 error, the carrier’s filtering system has probably recently identified a pattern in your messages that triggered a block.

Message Delivery in the United States and Canada FAQ

Q: I think my Twilio number or message contents may have been blocked by a carrier. Can I get the block removed?

A: No. Twilio is unable to request any unblocking of phone numbers or message bodies due to filtering, unless the messaging in question is true Person-to-Person (P2P) messaging and the block was in error (this is uncommon).

Most carrier block lists in the U.S. and Canada for numbers use a “cooling off” period, which means that the numbers will automatically be removed from the blacklist after a period of time. The time period may vary, and carriers do not share this information with Twilio. If your message bodies do not also change, carriers’ content filtering systems will continue blocking your messages.

Can I get my messages whitelisted or pre-approved by carriers?

U.S. and Canada carriers do not whitelist messages from long code numbers. Short code numbers, on the other hand, are whitelisted for a particular type of pre-approved traffic. If you are sending many messages with identical content to a large number of users, you are at high risk of having your messages filtered by carriers when using long codes, and should consider a short code.



This overview should help to give you a better understanding of how wireless carrier filtering works for Twilio Programmable SMS messages. For more information, please see the following resources:

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