JQuery :: How To Execute GetJSON Callback Function

I'm trying to execute a simple JSON request using getJSON but I can't get a successful response. My code is below. If I run this I always receive an error. If I add "callback?" (making it JSONP) then I can see the correct results in Firebug but the call back function is never executed.
function getResults(url) {
var gptUrl = "[URL]";
$.getJSON(url, {f: "json"}, function() {
alert("success");
})
.success(function() { alert("second success"); })
.error(function() { alert("error"); })
.complete(function() { alert("complete"); });}