The external system is sending XML wrapped with SOAP. I created a flow service, created a provider web service and provided SOAP endpoint address to external system to do HTTP POST.
Below is the Message structure they are sending to webMethods using HTTP POST.
You may use soap over JMS. You can receive that XML and extract SOAP message as is. After that publish that to JMS provider to subscribe it using soap trigger.
I believe this will not work.
They have to consume your webService and hit the same URL.
Also, if they hit your webService URL with their own SOAP xml structure i believe our IS will not parse as expected. It might be because of issue with document styles.