Tony hits the nail on the head
What is SOAP? It’s an HTTP POST containing XML, and an HTTP response containing XML. You can make an echo server by – here it is – simply serving a static XML file. That’s it. Just put an XML file on your http server and you’re done.
Alternately, if you feel the need, you can POST the content to the echo service. If you want to wrap it in SOAP too, even that’d be fine. Just watch that immediate child of soapenv:Body, because what you ABSOLUTELY do not need, is an “echo operation“. You just need the content to be echoed, and the address of the service; document in, document out.
That’s document exchange. That’s REST.
Can we get some real work done now, please?