AJAX calling an external webservice from the client

I'm going to use an external WebService which check if an adress exist. A year ago I tested using webservice a little so I shouldn't be quite unfamilar with that subject but . . . So far I have created a webservice client subclass from the WSDL. Now I'm stuck about how to invoke the external webservice. The client/user type in a new address on a page that use VDF AJAX but the address has to be validated before a save may be performed.