I was working with Siebel webservices for Query Customer and found that it was working fine from BPEL but when I try from soapUI, it returns junk characters. Upon intercepting both traffic using tcpmon, we found the issue was in one of the HTTP header. Basically "Accept-Encoding: gzip,deflate" which was sent from SoapUI was causing the problem.
In soapUI, you can disable this particular header via following option. Once the option is enabled, you can invoke Siebel web service doesn't return junk data and returns valid XML data.