Elastix

It's a very complete full featured opensource PBX. Does voicemail to email, fax 2 email, conference rooms, presence, reporting, call recording, etc. Can connect to SIP, PRI, Analog, whatever you have a device for. It's very very cool. It's basically a full OS installer that installs Centos Linux, Asterisk PBX software, Custom web interface with a FreePBX like GUI with some additional features. It can also do office email, instant messenger, vtiger CRM, firewall, etc. But the phone stuff is basically all I've ever used it for.

And it's totally free with no licenses needed to do anything. There are some extra commercial modules, but I've never used them.

It is Linux, but beyond the install of the OS you never see it. Everything is done through the web. If you don't like to install Linux or your lazy like me can buy a box preconfigured with Elastix on it http://www.voipsupply.com/elastix-miniucs-appliance . You can also pay for commercial support which is sometimes rare with Open Source stuff.

As of 2016, Elastix has been acquired by 3CX and is no longer open source. Version 2.5 is the last FOSS version, and everything from 5.0 is proprietary.

They do offer free keys for limited users (testing) but will charge beyond the basic amount. Very disappointing.

Elastix is not bad at all, I just had problems with Linux operating system. I dont prefer, its very complex for me.
I like working on Windows.

I came across a telephone system, Ozeki Phone System XE that allows me to connect my old Elastix system to it. I was very glad to hear that this is possible because I didnt have to throw it away.
Ozeki Phone System XE is more modern and easier telephone system, try it. http://www.ozekiphone.com/elastix-vs-ozeki-pbx-405.html