This guide discusses SMS filtering by wireless carriers that can affect delivery of your BatchLeads (BL) SMS messages. This is often done to comply with state or local regulations, or wireless carriers’ messaging policies.

Filtering of your outgoing BL SMS messages can vary widely between carriers. While this can seem like a black box at times, we have compiled the following information to help you understand why a message might have been filtered.

Why do Wireless Carriers have Filtering Systems?

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 country, laws or regulations may put restrictions on certain types of messaging, or even forbid certain things altogether.

For example: In the U.S., 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. As of August 2019, the only exception is Verizon Wireless with their 10DLC A2P Long code product. Other carriers are soon to follow.

2. Protecting mobile subscribers from unwanted messaging such as spam, fraud, or abuse

When mobile subscribers receive messages they find objectionable, they may file complaints, seek damages, or simply stop being a customer. All of these things reduce revenue or increase costs for carriers. Thus, it is in the best interest of carriers to protect mobile subscribers from what they consider to be objectionable content.

How do Wireless Carriers Filter Messages?

There is no standard practice for carrier filtering across all carriers. Filtering can range from a simple static list of prohibited terms, to advanced machine learning systems that constantly adapt based on the messages passing through them. Regardless of the system, carriers keep their filtering systems closely guarded secrets. Due to the ambiguous nature of these filtering mechanisms, BL 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?

BL can't predict filtering, and does not always know for certain when a message has been filtered by a carrier. Some carriers may falsely report filtered messages as delivered to prevent spammers from reverse engineering filtering systems. In other cases, carriers will tell BL that a particular message has been filtered. The error code we will use when we know that carrier filtering has taken place is for example ERROR 3009: Message Delivery - Carrier Violation.

How do I Prevent my Messages from Being Filtered?

BL works hard to keep our customers informed about the rules and regulations affecting SMS sending/receiving.

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

Provide a good user experience: Users 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 contents) will be filtered. In some cases, the user may have forgotten that they requested the message, so be sure to identify your business or service in the message. Also, how the message is formatted and written is important. Overly long messages, overly capitalized messages, mysterious links, hyperbole, and using aggressive language can make users suspicious of a message.

If users do not understand how to opt out (unsubscribe) from your messages, they may feel they have no choice but to contact their carrier to request the carrier block your messages.

Filtering Mechanisms

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

Message Delivery in the United States FAQ

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

A: No. BatchLeads is unable to request any unblocking of phone numbers or message bodies due to filtering.

Most carrier block lists in the U.S. 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 BatchLeads. If your message bodies do not also change, carriers’ content filtering systems will continue blocking your messages.