I'm not an ABAP programmer.
If I have to test a webservice, provided by sap system or other system, I always use soapui from www.soapui.org
In soapui
* create new soap project
* point to wsdl file
This will automatically create a sample xml request message for the webservice.
You can edit the request message and put in the values you want.
If required, fill user credentials into the request properties.
Then send the request message.
You will get the xml response message.
If the service works fine with soapui, then the problem is at your side (abap proxy, apap code behind the proxy).
If you can reproduce the error with soapui, then the problem is at the side of the service provider.
I don't have permission for SPROXY at our sap backends. Perhaps there is a "test interface" function, which shows the xml messages.