“FusionPBX is an open source FreeSWITCH GUI (Graphical User Interface). FusionPBX can be used as a high available single or domain based multi-tenant PBX, carrier grade switch, call center server, fax server, voip server, voicemail server, conference server, voice application server, appliance framework and more. FreeSWITCH™ is a highly scalable, multi-threaded, multi-platform communication platform. FusionPBX license is the MPL 1.1 license.
It can be run on most operating systems on the hardware of your choice. It provides unlimited extensions, voicemail-to-email, music on hold, call parking, call center, call queues, analog lines or high density T1/E1 circuits and many other features. It provides the functionality your business needs and brings corporate level phone system features to small, medium and large businesses.” Source: http://www.fusionpbx.com/