Your message was filtered (blocked) by Twilio or by the carrier. This may be done by Twilio for violating Twilio's Messaging Policy or Acceptable Use Policy, or by a wireless carrier for violating carrier rules or regulations.
Examples of messaging that would be blocked by Twilio are spam, phishing, and fraud. Twilio's filtering system is in place to protect mobile subscribers from spam or other forms of malicious or unwanted messages.
Wireless carriers have filtering systems in place as well. These carrier filters are used to block abusive traffic, as well as to enforce rules or regulations about what types of messaging are allowed to that country or mobile network.
Learn more about how message filtering works and how to avoid it: How Does Message Filtering Work?
For a detailed explanation of carrier filtering in the United States and Canada, please see SMS Carrier Filtering in the United States and Canada.