ASP.NET AJAX call to async WebMethod does not return

In my web app I am making a AJAX call to an asynchronous webmethod that does not return.

Default.aspx has async="True"

The asp script:

$.ajax({
    type: "POST",
    url: "Default.aspx/Process",
    data: params,
    contentType: "application/json; charset=utf-8",
    success: function (data) {
         $("#AjaxDiv").text(data.d);
    },
    error: function(textStatus, errorThrown) {
          $("#AjaxDiv").text(textStatus.responseText);
          alert.text(textStatus.responseText);
    }  
});

On the server, the code is:

[WebMethod]
public static async Task<string> Process(int id)
{
    var data = await DataInterface.Call(id);
    return data.ToString();
}

However, DataInterface.Call() never returns, although via the debugger, I can see that Call() does complete. What settings or attributes might I be missing? whether it be in the app/web configs or page settings.