run startmodem.
Why not just go to /etc/ppp and run the setup script from in there surely if you have the single modem it'll configure. I've never used a usb modem for red hat but I'll give you a link and hopefully it'll help you out its for Red Hat help section so thats all I can suggest I can only suggest setting up the script and doing that.
e.g.
ifup ppp0 would automatically run the script.
maybe I'm off try but anyways.
Here's the link for all the documenation.
http://www.redhat.com/docs/manuals/linux/
HTH
Marc