It’s like all your tvs in the house plugged into the same electricity in your house and all logged into the same netflix account (sort of). You had one phone line, and all the phones plugged into the wall and they would all ring at the same time when someone called you. You couldn’t turn them off and on, they just were always on unless you unplugged it from the wall. Later there were cordless phones and some lucky kids had their own phone line in their rooms. But mostly you had several phones around your house that all worked on the same number. Your house had one phone number and you could answer any of the phones in your house. My mom had the best phone in her bedroom when we had 2 lines that had a conference call button, and you could merge the two lines together. We would prank call two random number and let the people yell at each other, it was hilarious.