Carrier Filtering Overview

Overview

If too many text messages are sent to mobile numbers hosted by the same carrier (e.g., Verizon, AT&T, etc.), the recipient's carrier may begin to reject text messages from the sender's number due to spam filtering. Individual messages may be flagged as "carrier violations" as well, depending on the content of the text message. This is actually an intentional feature, implemented by the carriers, to prevent mobile numbers from unwanted spamming.

How do I know if my messages are being filtered?

Cadence's SMS provider Twilio does not always know when a message has been filtered by a carrier. Some carriers falsely report filtered messages as delivered to prevent spammers from reverse engineering filtering systems, while others tell Twilio that a particular message has been filtered. If Twilio knows that a message has been filtered, we will pass this information to your application by sending a request to the status callback URL you set when you sent the message.

An error code of 30007 indicates that it has been verified that carrier filtering has taken place.

There are other error codes, such as 30003, which have a more generic error message, but are often also associated with carrier filtering. In these cases, the carriers do not want to indicate to the sender that they are performing filtering.

Why do carriers have content and spam filters?

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

How do carriers filter messages?

There is no standardized practice for carrier filtering and the parameters associated with filtering vary on a carrier-to-carrier basis. In turn, Twilio cannot say definitively how these systems work or why a particular message was filtered.

Links (Updated 07/08/2020)

Based on new information and from observing carrier behavior, including a link in a message is now at a high risk of that message being filtered and/or not delivered. This applies even to single messages not sent as part of a batch (including Smart Messages). Shortened links, using services such as bit.ly and goo.gl are filtered at a higher rate than messages containing standard, full links - but both will put the delivery of your message at risk. We have also noticed that including emails in a text message have begun to be flagged (emails are being considered links). For this reason, links should be avoided. This also includes email addresses, which also should not be sent in text messages, as most carriers read them as links.

In some cases, messages that include a link are not delivered at all to the recipient and do not display a carrier violation code/not delivered message due to carriers not reporting the message was filtered.

How do I prevent my messages from being filtered?

When using Cadence, there are several steps you can take to reduce your chances of a carrier violation. These steps include:

Avoid sending mass texts from the same phone number to multiple lists of contacts in a short time period.
As the amount of unique contacts that are being texted per day by a single number increases, so does the risk of carrier filtering.
Avoid sending links if possible - shortened links specifically, because they hide the actual destination, and full links put a message at a much higher risk of filtering by certain carriers
Utilize the "Send as assigned user" function (Admins only). This function will send text messages from each users texting number (rather than one single texting number), therefore reducing the amount of messages that are sending per phone number in Cadence.
Avoid confusing messages. Suspicious recipients are more likely to report messages to their carrier, and when messages are reported to carriers, it becomes more likely that future messages from that number or with similar content will be filtered.
Avoid overly long messages, messages IN ALL CAPS, short links, hyperbole, and the use of aggressive language, as these may raise the level of suspicion of your recipients.

Filtering Mechanisms

Carriers in the United States and Canada utilize adaptive (machine learning) software systems to protect their users. These systems take into account both the rate of send as well as the content of the messages 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 time period, how many similar messages have transited the carrier’s network, and if the message contains content that makes it a high match for spam. Time periods are measured by the second, minute, hour, and day.

Cadence and Long Codes

Cadence users utilize 10-digit long code phone numbers to send text messages. Twilio does not definitively know how many messages can be sent from a long code before a user can expect to hit a filter, however, we are sure that sending more than one message per-second, per-number will cause a message to be blocked.

To prevent this, Twilio will queue messages and release them at this rate to protect users from inadvertently getting their numbers blacklisted.

Beyond that, we have seen carriers blacklist numbers after users try to send more than five messages per-minute or more than 250 messages per-day, but this is not a rule and does not happen to all users.

Carriers in the United States sometimes 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 has taken some sort of action which has caused your message to be filtered.

Lastly, carriers prefer no more than 200 recipients per day per long code phone number in order to avoid carrier violations.

FAQ

I think my number has been blacklisted by a carrier. Can I get it removed?

No. However most blacklists in the United States and Canada use a “cooling off” period, which means that most numbers will automatically be removed from the blacklist after a period of time. This period of time varies based on how many messages were blocked by the carrier from the number, and carriers do not share this time period with Cadence.

Can I get my messages whitelisted by the carriers?

Carriers in the United States and Canada do not whitelist messages from long code numbers. Short codes are essentially numbers which have been 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 and should consider a short code.

How do I know I am sending to Actual Mobile Numbers?

If you are unsure whether or not the phone numbers you are texting are actual mobile numbers, you can enable Phone Number Validation. Additional details can be found here:

Phone Number Validation