Product: Programmable SMS
You have attempted to send to a number that is not currently reachable via Twilio SMS. If the number provided is a properly formatted E.164, it is most likely that Twilio does not yet have service with the carrier you are trying to reach. We have logged the carrier you are attempting to reach and will monitor these failures when adding new carriers. The combination of “To” and “From” parameters on this message cannot be connected by Twilio at this time.
Sender ID restrictions in the destination country: Many countries limit which numbers, short-codes, and/or alphanumeric senders can be used in that region. Consult the SMS guidelines for the destination region.
Sender ID restrictions on the destination network: Some networks do not support receiving messages from shortcodes, others might allow SMS from shortcodes but cannot receive MMS messages from shortcodes. Similar restrictions may apply to the use of longcodes from countries that do not match the destination country. Consult the SMS guidelines for the destination region.
Alphanumeric senderIDs: If you are using an alphanumeric sender ID, the 'To' number must be in a country where alphanumeric sender IDs are supported. Certain countries require pre-registration of alphanumeric sender IDs. A list of countries where alphanumeric sender ID is supported and whether or not pre-registration is required can be found here.
Number formatting: The format you used for the “To” or “From” number may not be formatted using the E.164 format. Twilio standardizes numbers using the E.164 format. If the “To” or “From” number of this message were altered by Twilio in a way that you didn’t expect, try re-submitting the message with E.164 formatted addresses.
Destinations where Twilio does not have connectivity: It is possible that Twilio does not yet have service with the carrier you are trying to reach. You can lookup the destination network via the lookup api.
Consult the linked documentation for each cause. You can also try sending again with a different "To" and "From" combination.