I don't think lots of people do still use dial-up modems, but I do have one, so I might as well get it working under my old Debian Woody running at my even older AcerAcros 486.
To be able to comunicate with the bloody modem, one individual should use “setserial” (my contemporary Debian doesn't have such command, so maybe it's a relic from the ancient age):
setserial /dev/ttyS3 irq <irq_number> uart 16550A
This supposes your modem sits at COM4, but maybe it's at COM3, I don't know… There are some jumpers for setting up which IRQ and COM port it takes. Make sure it doesn't conflit with other COM ports.
Note to myself: mine is at IRQ 7 and COM4.
After that, you can screen into it or even invoke ppp utilities to get online (wouldn't recomend, though.)