SMS Carrier Filtering

How Does Carrier Filtering Work?

In this article we discusses SMS filtering that is done by wireless carriers that can affect your delivery rate when sending SMS using Ytel API.

Carriers can and do block or "Filter" out text messages that are going through their network based on a variety of reasons. While carriers do not share this information with Ytel, the information below is designed to help you better understand why a message might have been filtered and how to minimize your messages from being filtered.

Wireless carriers have filtering systems for two reasons:

  1. Enforcing rules or regulations about what types of messaging are allowed to that country or mobile network. Depending on the laws or regulations, carriers may put restrictions on certain types of messaging that may include forbidden words or messaging content.
  • For example: In the U.S. and Canada, application-to-person (A2P) type messaging is generally not allowed to be sent using local 10-digit long code phone numbers. A2P traffic is supposed to be sent using short codes, and may be subject to filtering when sent via long code.
  1. Protecting mobile subscribers from unwanted messaging such as spam, fraud, or abuse

When mobile subscribers receive messages they may not want or do not remember opting in for, they may file complaints or simply stop being a customer. All of these things may reduce revenue and increase costs for carriers. Thus, it is in the best interest of carriers to protect mobile subscribers from what they consider to be unwanted messages.

How do Wireless Carriers Filter Messages?

There is no standard practice for carrier filtering across all carriers. Filtering can range from a single static list of prohibited words, to advanced machine learning systems that constantly adapt based on the messages passing through them. Regardless of the system, carriers keep their filtering systems hidden and do not provide details to others. Due to the nature of these filtering mechanisms, Ytel is unable to say definitively how these systems work, or why a particular message was filtered.

How do I Know if my Messages are Being Filtered?

Ytel can't predict filtering, and does not always know for certain when a message has been filtered by a carrier.

When sending SMS using our API, you can set the "Delivery Status" to "True". This will provide you with a response back IF we receive notification from our peering carriers about SMS delivery. However, not all DLRs guarantee that the target received your message. Some delivery receipts represent successful completion of just one stage in the delivery process, such as passing the message to another carrier. Other delivery receipts are fakes. Because of this, we cannot guarantee that a DLR is accurate.

You can also greatly reduce the risk of filtering by following these best practices:

  1. Identify your Business and Service in the message:
    Customers are likely to report confusing or unwanted messages to their carrier. When these messages are reported to carriers, it becomes very likely that future messages from the same number (or those with similar content or words) will be filtered. In some cases, the customer may have forgotten that they requested the message. Also, how the message is formatted and written is important. Overly long messages, capitalized messages, unknown links, and using inappropriate or misleading language can make Customers suspicious of a message.

  2. Provide clear opt-out instructions:
    If customers do not know how to opt out (unsubscribe) from your messages, they are more likely to contact their carrier to request the carrier to block your messages. If your messaging receives a number of complaints, the carrier may block your messages entirely or your phone numbers you are sending the messages from.

Please see SMS Best Practices and the CTIA for best practices when sending out Text messages to your customers.

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