It depends on the phone system type. For instance, Key System Units (KSUs) are physical units that contain various buttons, while each button represents a phone line.  The call agent picks up the receiver and presses the button corresponding with the line they want to select. To switch calls, an agent must manually route the call to a central calling hub, and the switching will take place there.  In contrast, Private Branch Exchanges handle this automatically, using programmable switching devices.  Meanwhile, VoIP systems deal with everything on the internet. Think of them as business-oriented versions of WhatsApp or Viber. You can call anyone, from everywhere, on any device without needing dedicated, on-premise phone systems.  VoIP systems include all the functionalities you would expect from a business phone system, like call transferring, routing, etc.