Long codes are traditional 10-digit phone numbers used for both voice calls and SMS in the U.S. and Canada. With the introduction of 10DLC (10-Digit Long Code) in the United States, local long codes can now be used for Application-to-Person (A2P) messaging, provided they are properly registered, follow carrier guidelines, and comply with messaging regulations. However, carriers will still block messages under certain circumstances.
Common Reasons for SMS Blocking
Carriers may block SMS messages if they detect patterns or content that violate their policies or industry guidelines. Some common reasons include:
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Non-Compliance with 10DLC Registration and Guidelines:
Messages sent from unregistered or improperly configured 10DLC numbers may be blocked. Ensure that your A2P campaign is registered and complies with carrier rules. -
Excessive Sending Rates Without Proper Registration:
Sending a high volume of messages too quickly from a long code without adhering to your assigned 10DLC throughput may trigger blocking. -
Imbalanced Incoming-to-Outgoing Ratios:
If the ratio of incoming messages to outgoing messages on your long code exceeds acceptable thresholds (for example, a ratio higher than 1:3), carriers may flag and block your messages. -
Repeated or Rapid-Fire Messaging:
Sending the same message multiple times to the same user within a few seconds can appear as spam and lead to blocking. -
Use of Prohibited Content:
Messages containing disallowed content—such as illegal offers, hate speech, phishing attempts, or other violations of carrier guidelines—will be blocked. -
Problematic Links and Keywords:
Messages that contain suspicious URLs, repetitive keywords, or link shorteners may raise spam flags and result in blocking. -
Multi-Unit (Multipart) Messages:
While allowed, longer messages split into multiple segments (multipart messages) may sometimes be scrutinized, increasing the risk of blocking if they resemble spam.
Note: When sending SMS to US destination numbers, specify your US Plivo long code number in E.164 format. If you don't, you'll get the error message "This caller ID is not allowed for outbound message.