The IP telephony systems are of the type of telephone facilities (voice communication) that exploit for the transport of the data IP packets.
In practice, the voice is "boxed" within IP packets and routed through an Internet-based network; this network can be local or connected to the Internet. Skype and other similar systems are examples of IP telephony.
It is good to know that by now all voice conversations take place digitally, using cellular technology, or the wired line (the wired telephone).
The elements of an IP telephone system are:
A telephone terminal, which can be: a) an IP phone, b) a software running on computer that emulates the telephone terminal, c) an adapter for connecting an analogue telephone.
Some wiring, which is normally shared with home or corporate data network.
A switchboard (PBX), which can be "physical" (within the premises), or "virtual" . The IP PBX is essentially a computer running a specific software (in Open Source environment Asterisk is used).
To connect to the outside world, there are two ways: using an IP telephony provider, or use a conversion interface with the traditional line