A technology based on IP technology,which allows voice/video calls,free IP to IP calls,voice conferencing etc, with cheaper prices:

Analogue system : (an old technology) central connected MDF termination with Cat3 cables.
Digital PABX : A technology similar to the analogue but has some add- ons basic feature (conference calls – view received numbers...).
VoIP : (Latest technology) voice is divided into packets and each one is set on an IP (features: video conference,applications,browsing from phone..)