Apparently, your SOAP request is not in format that your web service prefers.
SOAP faults are really just a placeholders for any kind of error reporting in which every error's format is purely dependent on given web service implementation.
With all this in mind,need to explore your web service implementation's error reporting and why it cannot unmarshall your request without telling you much more about it.
you can post your SOAP request and WSDL file that also describes the offending web service and i may be able to help you for the same.