Articles on: SMS/WhatsApp

SMS Filtering by Wireless Carriers


As with email providers (such as Google, Yahoo, etc.), mobile carriers often filter messages that they think are spam. However, rather than sending your message to the spam folder like the way email providers do, wireless carriers will not deliver your messages.

In this guide, we'll go over reasons for filtering SMS messages, what to check if you receive the Carrier Violation failure, and how to avoid SMS carrier filtering.

Why does SMS filtering exist?

SMS filtering exists for 2 main reasons. Understanding these reasons will help you determine how to avoid SMS carrier filtering in the first place.

- To safeguard mobile users. When customers opt-in to receive texts, they expect their carriers to safeguard their interest and privacy. Therefore, to deliver a good user experience and prevent users from receiving unwanted messages, mobile carriers use a filtering system. They also try to identify spam, abusive, and fraud SMS to filter troublesome text messages before they reach customers. This would help protect customers and loss of revenue as well.

- To comply with federal or local regulations. Countries follow texting regulations to send non-spam text messages to their prospects and customers. And carrier filters are one such element that helps block SMSes that violate these regulations. This way, it becomes simple for the carrier to comply with rules and regulations set by the government, as well as steer clear of potential legal action.

The way SMS messaging is filtered varies country by country and between different mobile networks. For example, in some countries (including the US and Canada), a long number (i.e., normal, 10-digit number) is often more susceptible to filtering when carriers believe it's being used for application-to-person (A2P) texting.

Many carriers also apply stricter filtering policies for long numbers than they do for short codes, although this situation is changing as SMS is gaining popularity. If you want to use a long number, it is recommended to have one long number per 200 contacts who have consented to SMS.

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

Related guide: SMS and MMS Prohibited Content

How do wireless carriers filter messages?

There is actually 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, it's hard to say exactly how these systems work, or why a specific message was filtered.

Receiving an SMS error message

As we said above, wireless carriers don't 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 AVADA as a Failed Delivery. Note that they won't always give a specific reason.

To view this information, navigate to an SMS campaign and the Activities section. You will see the Failed section right there, which shows recipients, message status, SMS text, delivery time, and error messages.

If you encounter this error, we recommend you run tests using phones on different wireless networks to see if you're really being filtered. You can also try:

- Sending you messages from a different phone 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 usually view third-party shorteners (i.e., as suspicious

How to avoid SMS carrier filtering?

Now we move to the meat of the guide: how to avoid SMS carrier filtering. Avoiding carrier filtering is not an exact science; it is an art. To lower the chances of your messages being filtered, you should:

Provide valuable content

Many SMS filters try to identify messages that they think customers won't like, such as unsolicited advertisements and coupons. If they see any messages containing similar characteristics to this type of content, they might pull it out.

To ensure your content is not mistaken as spam, make it valuable. For instance, customer service notifications, such as order notifications and appointment reminders, are helpful to customers. Meanwhile, overly long messages, overly capitalized messages, hyperbole, mysterious links, and aggressive language can make users suspicious of a message.

Send texts to small lists

When SMS carrier filters see a text sent to thousands of people, they assume that this is a "pray and spray" message. These types of messages are large, unsolicited advertisements, which often details some type of coupon or other deal.

In order to prevent the carrier from mistaking your message for spam, send your content to small lists. In addition to preventing carrier filtering, small lists help empower your teams to send targeted messages, which will likely elicit a favorable response from recipients. 

Update your lists and content regularly

Carriers might notice if the same message is repeatedly sent to many customers. To keep your content valuable, up-to-date, and fresh, you should continually update both your SMS lists and campaign content.

This will also ensure that customers don't get tired of seeing your texts land in their SMS inboxes. And, as always, make sure to provide clear opt-out instructions to ensure that recipients don't have to see your texts if they don't want to.

Other best practices

- Let recipients know who is sending the message

- Don't 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

- Use a personal tone to sound more human

- Avoid SHAFT-related words

- Use a trusted sending number

- If you are sending A2P traffic, the carriers in the country you are sending to may require that this traffic be sent from a short code or from a pre-registered Alphanumeric Sender ID, if available. These are both ways that carriers can review your messaging use case in advance, and offer better delivery as a result.

Read more: Countries supported with Alphanumeric SMS Sender ID

Updated on: 28/08/2021

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