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: