Traditional PBX comes with proprietary phones that are either vendor-lock-in (phones are only usable with systems from the same vendor) or system-lock-in (phones can only be used using the same system).
A recent advancement in this field is called Open-Standards-based IP-PBX, which allows users to make and receive calls using Internet Protocol (IP) as the underlying transport technology.
Unlike with a traditional PBX, IP-PBX allows for almost unlimited growth in terms of extension and trunks. It also offers more complex functions that are more costly and difficult to implement with a traditional PBX, such as ring groups, queues, digital receptionists, voicemail, and reporting.