21703: The Messaging Service does not have a phone number available to send a message

This error occurs when your Messaging Service has senders in its sender pool, but none of them can send the requested message or media to the recipient. When you send with MessagingServiceSid, Twilio accepts the request, selects a sender based on destination and sender capabilities, and fails the message if no eligible sender is available.

• You are sending to a United States or Canada mobile number, but your Messaging Service does not contain a United States or Canada phone number or short code. Twilio does not use phone numbers from outside the United States or Canada to reach mobile recipients in those countries.
• Your Messaging Service only contains a short code, and that short code cannot reach the recipient. This can happen when the recipient is in a different country from the short code or the carrier does not support short code messaging.
• You are sending MMS, but your Messaging Service does not contain a United States or Canada long code or an MMS-enabled short code. Twilio supports MMS on short codes only where MMS short code support is available.
• Your Messaging Service only contains an Alphanumeric Sender ID, and the destination country does not support Alphanumeric Sender ID. Some countries also require preregistration before you can send with that sender type.
• You are sending to a United States mobile number with a 10DLC sender that is not associated with a verified A2P campaign. Messaging Services used for A2P 10DLC campaigns must include the phone numbers you plan to use in the sender pool, and unregistered numbers are not associated with a verified campaign.
• You recently added a sender to the Messaging Service, but registration or number association is still pending. Twilio notes that newly added numbers can take time to become available for sending in some registration flows.

• Add a sender to the Messaging Service that supports the destination country and the message type you are sending. You can add phone numbers in Console or by using the Services PhoneNumbers API.
• If you send to recipients in the United States or Canada, add a compatible sender from that region. Do not rely on phone numbers from outside the United States or Canada for delivery to mobile numbers in those countries.
• If you send MMS, use a United States or Canada long code or enable MMS on your short code before sending media.
• If you use Alphanumeric Sender ID, confirm that the destination country supports it and complete preregistration when required. If you need fallback for unsupported countries, add a compatible phone number to the same Messaging Service.
• If you send to United States recipients over 10DLC, register your brand and campaign, associate the campaign with a Messaging Service, and add the phone numbers you plan to use to that Messaging Service before sending.
• If you recently added a sender and it is not yet available through the Messaging Service, send temporarily with an explicit From value that names an eligible sender instead of relying on MessagingServiceSid. Then retry the Messaging Service flow after registration is complete.

• Messaging Services
• Services PhoneNumbers subresource
• Programmable Messaging and A2P 10DLC