This is an application to access your Web Service/API without worrying to create your own client application for the sake of testing. Personally, I’m using soapUI (an open source application to test web service)

http://sourceforge.net/projects/soapui/files/

I’d prefer to use tool this because it provides you a transparent communication between the client and the Web Service and it’s really simple to use and configure. It’d save you a lot of time to debug the issue for your client. You can trace it down whether the issue is in the client or in the web service itself