PBX stands for Private Branch eXchange. In the past, due to a limited number of phone lines being connected to a building, the primary function of PBX was to allow internal calls between system users and for connection of more phones to the system than lines available. Lines were effectively shared amongst users via PBX. Modern technology does not require traditional PSTN lines and uses data links instead which is called Voice over IP (VoIP). PBX is still widely used, although its primary function is now to improve call handling and user performance via advanced call routing features. Modern cloud PBX is effectively software that controls digital voice data.