When sending WhatsApp messages, if a user has blocked your phone number in the WhatsApp app (see below), then your messages will not be delivered.
In the Twilio Console or via API, you can view the status for each message you send. Messages sent to blocked phone numbers will show as “Sent”. Please note that there’s other scenarios in which a message might stay in “Sent” and it does not guarantee that the user has blocked your WhatsApp Sender.
WhatsApp used to return a specific error code when sending messages to blocked phone numbers. This is no longer the case as they view this a violation of privacy for the end user. As a result, we are no longer able to pass that specific error code along.