I don't know anything about earthlink or netzero, but a dial-up provider typically has a system as follows:
1. Lots of modems. These can be "real" modems, or software within a digital switch (aka phone
exchange).
2. Port concentration. This takes all those modem calls and merges the data streams to something more useful: ethernet, fiber, some sort of internal backplane etc. and routes them to a port on the machine which runs the software in (3).
3. Listener software. This accepts incoming modem data connection attempts, validates and authenticates them (some aspects of this may be pushed forward into stages 1 and 2 in some systems) and decodes PPP (or whatever) into straight TCP/IP packets.
4. Routing. The TCP/IP packets are then sent to the destination using whatever network(s) the routing system chooses. Incoming packets for the modem-connected system are then routed back to the PPP software and encoded for dispatch to the remote machine.
If you want to set up a system like this yourself, you need a modem which will answer calls, and some PPP Listener software which will take incoming connections, convert them and forward the packets to whatever routing you have already set up. Software like this is readily available for Linux, and presumably you can also get it for other systems.