Two or more analog phones can go on the same line by simply putting them in parallel. They'll all ring when the ring voltage is applied. Any one of them that is picked up will short the ring line, telling the central office (or phone switch) that a reciever is off the hook. The voice pair of wires will continue to function as full duplex.
Digital phones won't work this way. They have to have a dedicated home run line to the phone switch. Ditto for V/IP and a server.