In case somebody wants to create ONE calendar (e.g. holidays, many moving holidays in Austria) and use it on several freePBX servers, here’s a short description.
OwnCloud is open source and has a calendar app. Some domain providers (like all-inkl.com in Germany) provide ownCloud ready-to-use software packages. You can also ssh in and install it manually. Once installed at your domain, you share your calendar (public, read-only).
Now, you send an invite email with the link to your calendar to your email address…and check if it works.
In freePBX you create a remote-iCal-calendar and use the URL you get once you follow this procedure:
You have to click on the link in the email. A calendar window opens in your browser. Now right-click on the download icon on the right upper corner and copy the link. That’s the link you have to enter in the freePBX calendar URL field (it says “…?export” at the end).
Now you have one online calendar, which controls several freePBX servers
EDIT: By the way…ownCloud also supports CardDAV (contacts app)…in addition to CalDAV…so you can sync all your devices (Iphones, Androids, MacOS, Windows, Linux, etc) and have all your contacts synced as well!