You have attempted to send to a number that is not currently reachable via Twilio SMS or MMS.
If the number provided is a properly formatted E.164, it is possible that Twilio does not yet have service with the carrier you are trying to reach.
If you are using an alphanumeric sender ID, the 'To' number is in a country where alphanumeric sender IDs are not supported. A list of countries where alphanumeric sender ID is supported can be found here.
We have logged the carrier you are attempting to reach and will monitor these failures when adding new carriers.
Other possible causes include:
- Sending international SMS messages from domestic only geographic phone numbers or short codes
- Sending MMS messages to an unreachable carrier with MMS Converter disabled.
- Sending from Globally SMS-enabled numbers is not supported when reaching +1 destinations (US and Canada). Use other +1 SMS-enabled Twilio numbers when communicating in these regions.