The Linux based Open Source Software Asterisk has become the de facto standard in modern VoIP PBX systems. Because of its powerful and flexible structure Asterisk is also being used as the VoIP engine in commercial PBX products, partly because some pbx manufacturers have realized that it would not make much sense to compete against the development momentum of this open source project and end up having an expensive look-alike that no one writes interface software for.
The flexibility of Asterisk comes with a price, though. There is no user friendly interface included and the command language and syntax have a very steep learning curve.
Even though some VoIP enthusiasts are configuring their Asterisk PBX from the command line interface, this is not practical for a commercial product. Managing a pbx system this way would be just as absurd as trying to sell a fax machine that needs a computer science diploma to operate.