Old Stockholm Telephone Tower

[1] https://en.wikipedia.org/wiki/Optical_telegraph

Using a switch, each connection is a literal physical circuit. Multiplexing allows multiple circuits per phone line (through frequency separation at the carrier-frequency level), and much early telephony research involved increasing the multiplexing capacity and consequent issues.

With packet-switched networks, the only circuits are the interconnects between routers, and each individual data packet can take a different route. Subject to quality of service / service level agreements (QoS / SLA), it's possible to support far more distinct individual connections over packet-switched networks, though there still remains a maximum total bandwidth. For time-sensitive modalities (e.g., realtime voice or video), excess traffic leads to congestion and buffering, distortion, or interference, so limits remain. But they're far more generous than with circuit-based networks.

Put another way, packets give a greater assurance of establishing a link between any two nodes, whilst circuits give a greater assurance of a minimum bandwidth floor between those nodes. If you can get a connection, circuit-based line quality is often (though not always) superior, in terms of consistency, clarity, and low latency.

Under PSTN, public-switched telephone networks, a not-infrequent occurrence, especially when calling long-distance, was to get a message "all circuits are busy". When each call was literally a circuit, and the "switch" (the central telephone exchange) made and broke those circuits as calls began and ended, my understanding (not my area of expertise, but one of some interest) is that this meant that all available interchange connections were occupied. For long-distance, this was typically far lower than for local calls (most phone traffic is local), and for international calls, lower still. The first transatlantic telephone cable could support only 36 simultaneous calls, in 1956. Calls were short, expensive, and all but exclusively for business and government subscribers.

<https://hamhistory.org/first-transatlantic-telephone-cable/>

I'd expect that the Stockholm exchange probably supported a few hundred simultaneous calls, probably a few (a dozen or so perhaps) per operator, who had to physically connect each call.

Still, pretty astronomical.

When you made a call, your local operator would have connected you either to a local number on their own board, or to another local board as needed. That second operator would have then connected you to the desired number.

Each board would only have limited connection lines to each other board (or to a branch exchange). So if all the connections from board A to board B (or to the branch exchange) were in use, the caller would have to try again later.

For some perspective here - it took until the mid-80s for most of Germany to be connected to a phone line. That is, the 1980s.

I recently talked about that with my father after I found a postcard from one of my uncles from the early 80s confirming meeting and dinner plans. While I remember them always having a phone they were one of the households only connected in the mid 80s - which in retrospect explains some of the things I've found odd about them when talking to them by phone. It was a new thing for them.

(My parents got connected early on - my mother used to work for the post office in the phone exchange, and one of the perks of the job was priority for getting a phone line. Which also explained why we had an old grey phone, while pretty much all my friends had a relatively modern - for the time - one: they all only somewhat recently got phones)