// alert message on some failure
function CallFailed(res, destCtrl) {
    alert('Call Failed : ' +res.get_message());
}

function SayHelloMethodCall(name, obj) {
    alert(name);
    PageMethods.SayHello(name, SayHelloMethodCallSuccess, CallFailed, obj);
}

function SayHelloMethodCallSuccess(res, obj, userNameUnavailableErrorMessage) {
    if (res != "") {
        alert('Response from WebMethod : '+res);
    } else {
        //incorrect response
    }
}