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.
ifup ppp0 would automatically run the script.
maybe I'm off try but anyways.
Here's the link for all the documenation.