A for instance was when the family lived in a 3xstorey house,we had 5 phones in all which were added by a friend who worked for BT.All on the same line.
I was upstairs,3rd floor,cleaning the phone and as I lifted the receiver,my late husband who was downstairs,picked up the phone and asked me if I needed anything as he'd heard the phone tinkle. No,I said,I was just cleaning it.
It was a Godsend when he was ill in bed because he'd use the phone in his bedroom to call down if he needed assistance. You only had to lift the receiver off its cradle,without any dialling,in order to speak to someone within the home. This is what you CAN do with multiple phones in the home and just one line coming in.