how does dialtone
get from the
phone company
to your desk?

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With older "1A2" electromechanical phone systems, there is usually a discrete copper wire path for each phone line from the phone company's central office, through the streets, then through your phone system's control unit, to the phone on your desk. You press a button ("key") on your phone to select which line passes through the phone base to the handset, and then you can talk.

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Simple multi-line phones and KSU-less systems work much the same way, but without a control unit.

 

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With a modern Panasonic hybrid phone system, pressing a button commands your control unit to connect you to a specific line.

Even though only one line is 'inside' your phone at any given time, you can instantly switch to another line, and set up conference calls. Each line button makes a momentary connection to set up a talking path, instead of being mechanically latched with a failure-prone linkage of levers and springs.

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In cases where the phone company doesn't have enough wires running around the streets, they can squeeze multiple talking circuits onto one pair of wires, using Subscriber Line Carrier equipment. The multiple lines are "concentrated" at the phone company end of the wire, and then separated in your home or business. Bigger SLC systems can carry 96 circuits on two pairs of wires, for a neighborhood or a large business.