By Jonny Lupsha, News Writer

The Federal Communications Commission (FCC) wants phone companies to block robocalls by default, according to NPR. Currently, customers must opt in to block specific unwanted calls. How did we get here from copper wires and switch systems?

In a statement released last week, the FCC said their proposal would allow phone companies to block robocalls by default and allow consumers to block calls from phone numbers not on their contact lists. Furthermore, the statement proposes a “safe harbor” for any phone providers who choose to implement network-wide bans on numbers that fail an authentication check under the upcoming regulation system known as “SHAKEN/STIR.” These developments stand in stark contrast to the more personal age of the landline telephone system.

What Your Phone Number Means

The public switched telephone network (or PTSN) refers to individual subscribers in houses as well as groups of subscribers in apartments or offices. They were connected to a Private Branch Exchange or PBX, which allowed communication between two members of the same group or between one member of the group and someone of a different group. “For both individual subscribers and the PBX, the gateway to the outside world is a higher-level switching system called the local exchange, housed within a facility called the central office,” said Dr. Stephen Ressler, Professor Emeritus from the United States Military Academy at West Point.

The local exchange was a group of human telephone operators in a room of switchboards. When a call came in, the operator would ask how he or she could connect your call and physically connect pairs of wires into different corresponding input/output jacks based on your request. When the system became automated, we adapted 10-digit phone numbers. “In the U.S., every local exchange is assigned a three-digit code, which corresponds to the fourth, fifth, and sixth digits of your 10-digit telephone number,” Dr. Ressler said. For example, if your phone number were (555) 123-4567, the 123 portion serves as your local exchange. Given the number of phones in the country, and the limitations of three digits, local exchanges often must repeat, which is where unique area codes come in—and why you didn’t used to need them when making a local call. Dialing the area code first bumps your call up to what’s called a “tandem exchange,” which connects to the call recipient’s tandem exchange and back down to their local exchange. The final four digits of your phone number are your subscriber number based on your local exchange.

Connecting Touch-Tone Calls

Once rotary phones had worn out their welcome, they began to be phased out and replaced by touch-tone phones—which bear a slight similarity to the virtual keypads we see on cell phones today. But how did it work? “Touch-tone dialing uses a system called dual-tone multi-frequency signaling,” Dr. Ressler said. “In this system, a different audio frequency is assigned to each row and column of the standard four-by-three telephone keypad. When you press any button, your phone generates a tone consisting of the two associated frequencies—think of it as a musical chord composed of two different notes.”

This series of chords is then decoded by a “digital signal processor” to know which buttons you pressed. This digital signal processor is the automated equivalent of the human operator who used to connect pairs of wires at a switchboard when callers lifted their receivers and rang out to make a call. It checks to see if the call recipient’s phone is off the hook and, if it is, either sends an audible signal to the recipient’s call waiting feature or it sends you back a busy signal. “If the recipient’s phone is on-hook, the exchange rings her phone by sending an AC current down her line,” Dr. Ressler said.

Nowadays, anyone with a smartphone can walk around wire-free and even hands-free to make phone calls, although we remain at the mercy of satellite signals being sent to and received from outer space. The FCC’s push to make our phone providers responsible for blocking automated sales calls may or may not be enacted, but such abstract problems facing the communications industry invite a look back on simpler times.

Dr. Stephen Ressler contributed to this article. Dr. Ressler is Professor Emeritus from the United States Military Academy at West Point. He earned a B.S. from West Point and an M.S. and a Ph.D. in Civil Engineering from Lehigh University, as well as a Master of Strategic Studies from the U.S. Army War College.