When your application is initiated to make a call, Plivo makes an HTTP request to the answer_url, which is one of the mandatory parameters when making an outbound call.
When your application is initiated to send a message, Plivo makes an HTTP request to the message_url, which is one of the mandatory parameters when sending a message.
answer_url or the message_url should respond with an XML document that provides instructions to control the call.Content Type of the response header, returned by the message_url, must be set to text/xml or application/xml.