FusionPBX is an open source FreeSWITCH GUI. 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. FreeSWITCHtm 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.