<?xml version="1.0" encoding="UTF-8"?>
<Response>
  <Message src="12023222222" dst="15671234567" type="sms" callbackUrl="http://foo.com/sms_status/" callbackMethod="POST">
    Hi, Message from Plivo
  </Message>
</Response>

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The parent element

The <Response> element is the parent element of Plivo’s XML. All child elements must be nested within this element. Any other structure is considered invalid.

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Child elements

Child elements are proprietary Plivo elements and are case-sensitive. This means that using <message> instead of <Message>, for example, will result in an error. Attributes for the child elements are also case-sensitive and “camelCased.” When Plivo receives an XML response, it executes the elements from top to bottom. 
<?xml version="1.0" encoding="UTF-8"?>
<Response>
  <Message src="12023222222" dst="15671234567" type="sms" callbackUrl="http://foo.com/sms_status/" callbackMethod="POST">
    Hi, Message from Plivo
  </Message>
</Response>