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