Error 21703

ERROR - 21703

None of the senders or numbers in your Messaging Service are capable of sending the requested message to this recipient.

Possible causes vary depending on the type of message and the country of the destination number. Below are several common causes of this error.

You attempted to send a message to a United States or Canada mobile number, but you do not have any US/Canada numbers or short codes in your Messaging Service. US/Canada mobile numbers are not reachable from Twilio numbers from outside the US/Canada, due to limitations imposed by carriers.
You only have a short code number in your Messaging Service, and the recipient is not reachable from your short code (for example, the To number is from a different country than your short code, or is on a carrier that does not support short code messaging).
You attempted to send an MMS message, but you do not have any US/Canada long code numbers or an MMS-enabled short code in your Messaging Service.
You only have an Alphanumeric Sender ID in your Messaging Service, and the To number is in a country where Alphanumeric Sender ID is unsupported.
You attempted to send a message to a United States mobile number, but you do not have any long code numbers that are registered with a valid campaign. Only numbers registered for a campaign can be allowed to send to US based numbers.

Add a sender to your Messaging Service with the required capabilities, using the Twilio Console or the REST API.