First, it should be said that this is a lost and dying art. Fewer and fewer ISP's are willing to support dial up modems, and the price (and performance) of a broadband connection like cable and DSL make that overwhelmingly the way to go.
However the dyed in the wool cheapskate or the person with truly limited means may find themselves given a hand-me-down modem, and may find a bargain from a dial up internet provider. So away we go.
Right now, I have in my hands a PCI Rockwell 56K modem. I have had good luck with Rockwell hardware modems in the past, and bought this (for $2.00) with full confidence it would be a hardware modem and easy to get to work. It is not a hardware modem. It is a "winmodem" and Rockwell is now Conexant. Plugged into a handy computer and poked at with lspci it shows up at PCI vendor ID 127A, PCI device 1003. 127A is one of the new PCI vendor ID's assigned to Conexant (the old rockwell id's for this thing were 2003,4,5, and 6)
Here are some links:
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