Are you doing Java development on the mobile? or are you using the BlackBerry Plugin for Visual Studio...
If you are using the Visual Studio plugin you can simply add a web reference to your Blackberry project and you are set to go.
If you are using the Java route... a simple way of doing it is just call your web service via an HTTP request sending down the envelope formatted to call your method. Then parse the response in the XML parser.
Similar to the way you would call it in AJAX Call SOAP Web services with Ajax, Part 1: Build the Web services client