Unfortunately the scanmodem program indicates this is not a well supported modem in slackware with the 2.6 kernel series.
You have a couple of options:
1) Try the above rmp@tgz conversions and pray.
2) Use the stock 2.4 kernel and use this
driver
3) Use an older distro as described on the above page.
4) Get a differrent modem. I have had good luck with modem based on the lucent/agere LTmodem chipset. I have found them new on the internet for around $9.00 US + shipping.