But we want to be more explicit, defining this as a SOAP RPC call. We can also define namespaces for the message in the Envelope itself. Making the namespaces explicit is not necessary, but can aid in [human] readability.

SOAP Envelope
<SOAP-ENV:Envelope
  xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/"
  xmlns:method="http://soapinterop.org/"
  SOAP-ENV:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">
  
  <SOAP-ENV:Body>

    <method:echoString>
      <inputString>hello world!</inputString>
    </method:echoString>
    
  </SOAP-ENV:Body>
  
</SOAP-ENV:Envelope>