The Web Callback module allows you to easily add an HTML call-me box to your web site. Visitors enter their phone number into the field in order to be connected with a specific destination. The Web Callback module allows you to prepend information to the caller ID, which can alert you that the call is coming from a specific callback. You can also set dial matches to specify which numbers users are allowed to call. Once you have set your destination and saved changes, you will receive HTML code to paste into your web site.
Since the Web CallBack module has you insert HTML into your web site, that has it pull files from the module to display on your webpage, you will need to make sure your web site has access to your PBX's /var/www/html/wcb.php file. This means your firewall cannot block it or any Apache security such as htaccess that you may have set up. Please note, PBXiaF by default blocks access to this without entering a password. You will need to modify their htaccess to allow this, and this is not something we can offer advice or support on.