I'm no very familiar with Red Hat, but I had a related problem after upgradind to Slackware 7.0 and using the KDE destop. I was starting pppd from /etc/rc.d/rc.local so kppp reported the modem as busy. If I killed the pppd daemon, then kppp wold work. starting ppp wit the -d option suited my needs even bette, providing demand dialing from the command line for things like Lynx. So I never did get the pap, login, user, password configured for kppp.
Also, if it's a PCI (maybe applicable tp PNP, but I skipped from jumpered hardware to PCI) there may be some differences which order the devices are detected/assigned resources. Look at the file /proc/pci. (Command line try cat /proc/pci | less, or any text editor, even an X text editor) This will tell you all your pci devices, but I hope you would recognize your modem by the brand or chipset name.