63015: Channel Sandbox can only send messages to phone numbers that have joined the Sandbox

This error occurs when you send a message from a channel sandbox to a recipient who has not joined that sandbox. In the WhatsApp Sandbox, you can send messages only to end users who have joined your specific sandbox. If the recipient joined more than three days ago, they must join again before you can send another sandbox message.

• The destination number has not joined your sandbox by sending join <your sandbox code> to the sandbox number.
• The recipient joined a different sandbox. End users receive messages only from the sandbox they joined.
• The recipient joined previously, but the sandbox session expired after three days and they need to rejoin.
• You are trying to use the sandbox to message a user who has not completed sandbox enrollment. Sandbox messaging is limited to testing participants only.

• Ask the recipient to send join <your sandbox code> to the sandbox number shown on your Sandbox page in Console. They can also scan the QR code and send the pre-populated message to join.
• Confirm that the recipient joined your sandbox, not another sandbox. If they switched sandboxes, have them send your current join code again.
• If more than three days have passed since the recipient joined, have them rejoin the sandbox before you send the next message.
• If you need to message users who have not joined a sandbox, move from the Sandbox to a WhatsApp-enabled sender for production messaging.

• Test WhatsApp messaging with the Sandbox
• Overview of the WhatsApp Business Platform with Twilio
• Register WhatsApp senders using Self Sign-up