Is the Xsd type for "ns0:EX_SOR_NUM" xs:int? If so, I would suspect your error is from validation.
If it's string, I would suspect you are receiving an Oracle error. Can you confirm and respond with any errors related to this issue?
I haven't had any luck calling functions directly, but sprocs within a package (Oracle 9i/10g) work fine. The Stored Procedure approach could be an option for you, but I'm not yet sure if you can achieve your desired results of passing a function directly. I'd like to try it myself and see if we can compare notes.