Why isn't jQuery validation plugin remote attribute working for me?

I'm using the jquery validation plugin and the remote attribute works with mailId but not mobileNo?

var validator = $("#addform").validate({
    rules: {
    Name: "required",
    MobileNo: {
             required: true,
           minlength: 10, remote: '<%=Url.Action("getClientMobNo", "Clients") %>'
                },
                Address: "required"
              },
   messages: {
       Name: "please provide a client name",
       MobileNo: {
          required: "Please provide a mobile phone no",
          rangelength: jQuery.format("Enter at least {0} characters"),
          remote: jQuery.format("This MobileNo is already in use")
        },
        Address: "please provide client address"
       },

      

        
        
        
      

    

The A value null

is being passed to my controller action. Any suggestion...

    public JsonResult getClientMobNo(string mobno)
    {
        JsonResult result = new JsonResult();
        string status = clirep.getClientMobNo(Convert.ToInt64(mobno));
        if (status == "Mobile No already exists")
        {
            result.Data = false;
        }
        else
        {
            result.Data = true;
        }
        return result;
    }