If you want to use the isapnp tools, then you do need a  conf file
You create an isapnp.conf file with the program pnpdump.  As root, type

/sbin/pnpdump > /etc/isapnp.conf

and then your file will be created.  It will consist of the isa pnp devices
you have in your system, and for each one there will be groups of possible
irq and io settings.

Next, edit the file and remove comments from lines so the settings match
your Windows setttings, but only disable the comments for one possible
group of settings.  Also, don't forget to uncomment the (ACT Y) or whatever
it is at the end of each section you are changing.

You can ignore the isapnp.conf stuff for any other cards detected
until later, as everything will be commented out until you remove the
comments.

Now you can edit /etc/isapnp.gone and comment out the settings for your
modem,
if they are the ones you will be using.  That way they are not reserved.

Now run /sbin/isapnp /etc/isapnp.conf and you'll see if the modem was
recognized.

You can then run setserial /dev/ttySx , with the x being the serial port you
are using.
This will show you your settings.  You may have to add a uart value, such as

setserial /dev/ttyS1 uart 16550A


    namuh
....
There is a file called  /etc/rc.boot/0setserial that you can edit.

Go down to the manual configuration section, take out the comment for
your modem, ie ttyS2, and edit the irq, io address, and uart type.

Those settings will stick the next time you reboot.