If you are one of those people who say that you will call home and let the telephone ring twice when you get to your destination safely, note that you have no guarantee that the telephone will actually ring twice—or even ring at all. You might hear two rings, but that does not mean the called party will hear two, or even any, rings because their power ringing pattern might be in an off period.
This is because the ringtone you hear in your ear when you're calling someone is generated at your local exchange, and is almost certainly out of sequence from the ring generator (called the "power ringing") at the far exchange. The circuit to the phone you're calling isn't completed until (if) someone answers. It's hard to believe, but those few seconds of leaving the circuit free (or not even using it if nobody answers) add up over thousands and millions of calls. Ultimately, it means the phone companies have to build fewer circuits to carry the same number of calls.
None of this is terribly important anymore ... cellphones have made the incremental cost of long distance calls so low (or even free) that we don't hesitate to call across the country to tell loved ones we arrived safely.
*SS7 stands for "Signaling System 7", the system which handles the setup and teardown of circuits for telephone calls. SS7 makes such things as caller ID and cellphone roaming possible. It is a fascinating system. Well, OK ... it is to me.
