People would get pissed if you didn't answer the phone right away, and might let it ring for 1/2 hr if they thought you were home. It was understood that part of being a good person was answering the phone within 7 rings. And to apologize if you didn't answer it until after the 7th ring.

I was taught you let it ring 7 times, unless they're elderly, then you go at least 10. And then answering machines came along, and within a few years, if the phone rang a 5th time you got confused.