In this article I will describe how it works if someone dials the telephone number +1-608-409-3188.
The person who dials this telephone number might be using a mobile phone or a landline phone or some other kind of phone. No matter what kind of phone they are using, they are making use of what is called “the PSTN” or the Public Switched Telephone Network. From its beginnings in the late 1800s until about ten years ago, the PSTN was mostly a worldwide network of physical facilities, mostly operated by traditional telephone companies. Most telephone customers were connected to the PSTN by means of “landlines” meaning a telephone line connected by a physical pair of copper wires connected to a telephone company central office. When you dialed a telephone number, quite often you were making physical things happen so that the telephone call would be connected to the destination. There would be clicking sounds in two telephone company buildings (one building near your location and another building near the location of the called party) to make the telephone call connect. Yes, actual clicking sounds because of actual physical things happening.
But by now the PSTN is mostly a cloud, very much like the Internet is mostly a cloud. These days when a person dials a telephone number, the very first thing that happens is that the call stops being physical and enters the first of many clouds. (We say that the call “is a voice over IP call” or “becomes a voice over IP call”.) The call passes through some clouds and eventually reaches the destination. At the destination the call again becomes a physical call and the telephone rings at the destination. A person picks up the phone and you hear them and they hear you.
The conversion from a physical telephone call to “the cloud” usually happens these days within the actual telephone that the person is using to place the call. The telephone connects to “the cloud” through an ethernet cable.
Even though we talk about everything being in “the cloud” (or, more accurately, in several clouds), common sense tells us that surely it must be possible to point to physical places where things happen. And that is what I will describe in this article.
When someone dials +1-608-409-3188, the call enters the PSTN. The caller might be anywhere, of course. They might be in the US or they might be in some other country. No matter where they are, their telephone carrier connects the call by looking up in a couple of databases to see where the connection point is for +1-608-409. The first database is the country-code database in which “1” means “the North American Numbering Plan” or NANP. Then the carrier goes to the NANP database and looks up “608-409”. They will find a pointer to a company called Big River Telephone Company in Arcadia, Wisconsin. Big River “owns” the ten thousand telephone numbers that are in the 608-409 exchange, having received this block of ten thousand telephone numbers in 2014. Normally what the carrier would do next is pass this call to Big River.
But then the carrier will do a further lookup to see that this particular telephone number has been “ported” to a company formerly called Level 3 Communications, now called Centurylink. So the carrier passes the call to Centurylink in a digital way. Centurylink looks up the ten-digit number 608-409-3188 and sees that (based upon instructions previously received from a company called VOIP.MS in Toronto, Canada) it should send the call to a building in Chicago, Illinois. The building is at 725 South Wells Street. It is a former printing plant that is now a major data center, inhabited by several “server farm” companies, one of which is called Steadfast. Steadfast rents a “cabinet” to VOIP.MS containing a telephone server.
The company whose phone number is 608-409-3188 has a telephone. At a previous time, the telephone had already established a “SIP trunk” which is a data link between the telephone and the telephone server in the cabinet in Chicago. The SIP trunk passes through an Internet cloud between the company (located in Madison, Wisconsin) and Chicago.
So what happens with this telephone call to +1-608-409-3188 is that the call reaches the server in the cabinet in Chicago. And then the call passes through the SIP trunk to the telephone. At the telephone, the call leaves “the cloud” and once again becomes a physical telephone call. And, if all goes well, when someone “picks up the handset”, the two parties can hear each other.
The astonishing thing is that all of these steps happen in perhaps 1½ seconds. From the moment that the caller pushes the green button on their cell phone, to the moment the call is ringing at the destination, might be only 1½ seconds.