there are various forms of that around - it’s a matter of your internal config and to some extent security issues. I had done something like that a long time ago (I’m sure it is somewhere) where I had an SBS Small Business Server as the external web so wrote a .NET script that did that, and then back behind the firewall on the trusted LAN side, triggered the FreePBX system to initiate the call. This keeps the FreePBX system off the web. No reason that external facing system could not be an Apache facing system and do a similar thing. If your Asterisk system happens to be web facing you can do it directly but that is usually not a good idea - having it act as an external web server…
as you can see though - it is going to depend on your webserver/FreePBX server setup, wan/lan setup, etc.