Description:
Installed PNP ISA modem was not recognized by system. Problem was in port conflict - PNP port x other ports. (The similar problem is also in Windows where modem cannot share IRQ3-4 with other ports. It's necessary to use manual configuration and assign IRQ5).

How to force PNP configuration:

• install isapnptools (RH7.2 CD2)
• determine IRQ,DMA and ports used and reserved to drivers and ports (COM,LPT). These ones cannot use PNP - write reserved values to /etc/isapnp.gone.
• dmesg | grep -i irq ... dump of all devices known to kernel
• read possible PNP configurations of PNP cards pnpdump > /etc/isapnp.conf
• customize this file - choose configuration, current config is in /proc/isapnp, allocated resources /proc/interrupt,pci,iomem,ioports
• now tell to kernel new COM port parameters. Insert lines into /etc/rc.d/rc.local

setserial /dev/ttyS2 irq 5 autoconfig
setserial /dev/ttyS2 spd_vhi
stty -echo < /dev/ttyS2

• add/change link /dev/modem to /dev/ttyS2
• reboot