gistfile1.js

var callbackform = new VarienForm('callback-form', true);
jQuery('#callback-form button').click(function () {
        if (!callbackform.validate()) {
          return;
        }
        var phonenumber = jQuery('#phonenumber').val();
        var callbackdiv = jQuery('#callback');
        var loadingImage = "<?php echo $this->getSkinUrl('images/loading.gif'); ?>";

        callbackdiv.html('<img src="'+loadingImage+'" /> Sending...');

        jQuery.ajax({
            type:   "POST",
            url:    "/bendart-callback/ajax/sendemail/",
            data:   'phonenumber='+phonenumber,
            success:function () {
                jQuery('#callback').text('Thanks! We will be in touch soon.');
            }
        });
        return false;
    })