Thanks for the suggestions everyone. To clarify, the goal of moving to a linux box or other appliance is two-fold. One is to get rid of the EOL cisco routers, the other is also to get rid of the shelf full of modems that are plugged into the routers, and this makes things a bit more complicated. There are a couple makers of internal modem cards that have tens or hundreds of modems integrated into one pci card, and this is where the need for a linux box comes in. Once all these modems are inside a server, I need to replicate the functionality of the cisco routers, which takes the raw serial input from each modem, and forwards it out over telnet to a predefined destination.
I've been working with mgetty the last couple days, and I've gotten pretty close to something that works. At the moment when you connect to my test pc's serial port, it will prompt for the username and then initiate a telnet session for you. Now how can I get mgetty to skip the login prompt entirely and go right to telnet? Any ideas?