The Web Callback module allows you to easily add an HTML call me box to your website. Visitors simply put there phone number in to be connected with you, or a specific destination.
Since the Web Callback module has you insert HTML into your website that has it pull files from the module to display on your webpage you will need to make sure your website has access to your PBX's /var/www/html/wcb.php file. This means your firewall can not block it or any apache security such as htaccess that you may have setup. Please note PBXiaF by default blocks access to this without entering a password and you will need to modify their htaccess to allow this and this is not something we can offer advice or support on.