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Computers allowed for the replacement of the long distance operators with computerized switches. The computers could create the connections and the billing records just like a human operator. Physical wires still connected you to the receiving party on each call, but the computer connected them together at each office. If, from Santa Monica, you dialed 1-212-555-1234 in New York, the 1 identified it as a long distance call, telling the local switch to connect to a long distance switch. The 212 told the long distance switch which long distance line to grab. Then the 555 told the long distance office in New York which local office to connect to. Then the local office would connect you to your friend. The computers in each office would pass the number along to one another as digital data via data lines connected between the switches.
In today's world, there are two things that make the system more interesting:
Physical wires no longer connect the offices together for each phone call. That system was incredibly expensive. Instead, a fiber optic line carries a digitized version of your voice. Your voice (along with thousands of others) becomes a stream of bytes flowing on a fiber optic line between offices. The difference in cost between "a pair of copper wires carrying a single conversation" and "a single fiber carrying thousands and thousands of conversations" is phenomenal.
The phone company is no longer a monopoly. Instead, there are many different long distance and local carriers.
Today when you place a long distance call, the switch in the local office accesses a database that contains a record for each phone number connected to the switch. The database contains what's called a PIC code (Primary Interchange Carrier code), which indicates which long distance carrier you have chosen. [When you switch long distance carriers, this PIC code is what changes.] The switch looks up the PIC code for your number and then connects to a long distance switch for your long distance carrier. Your long distance carrier's switches route the call to the local carrier for your friend, and the local carrier completes the call to your friend.
This entire amazing and complicated transaction happens using billions of dollars worth of computers, switches, wire and fiber optic cable in a blink of an eye!
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