jQuery UI autocomplete with JSON from URL

I'm using the jQuery UI Autocomplete function. I can make it work with the example provided with jQuery UI like this:

var availableTags = [
        "ActionScript",
        "AppleScript",
        "Asp",
        "BASIC",
        "C",
        "C++",
        "Clojure",
        "COBOL",
        "ColdFusion",
        "Erlang",
        "Fortran",
        "Groovy",
        "Haskell",
        "Java",
        "JavaScript",
        "Lisp",
        "Perl",
        "PHP",
        "Python",
        "Ruby",
        "Scala",
        "Scheme"
    ];

$("#tags").autocomplete({
    source: availableTags
});

This works without any problems. But I need to use JSON as my data source who can be retrieved like this: http://mysite.local/services/suggest.ashx?query=ball

If I'm going to that URL I get JSON back like this:

 [{"id":12,"phrase":"Ball"},{"id":16,"phrase":"Football"},{"id":17,"phrase":"Softball"}]

How can I use my URL as the data source?

I've tried changing the source-option like this:

$("#tags").autocomplete({
    source: "http://mysite.local/services/suggest.ashx"
});

But it doesn't help. I guess that the service doesn't know which keyword has been typed in the input field or so?

Any pointers would be great.