[kaerast]

When leaving my grandparents' house for a long journey home, everybody was always instructed to give three rings when they get home safely. To this day I always let a phone ring three times before answering, partly in case it's an "I'm OK" message and partly because waiting for somebody to answer gives the caller time to think what they actually want to say.

My grandparents didn't have caller ID and so had no idea who was telling them they'd got home safely, but they could guess based on timings pretty well.

[gwillen]

This works much less well in the days of digital call signalling. The "ringback tone" you hear -- the ringing in your earpiece when you call someone -- no longer corresponds in any way to the ring being generated on the other end. Ringback can start before or after ringing, so you can't even count on the number of seconds being the same, let alone the number of rings.