As long as you can log into the machine you dial into, there should be no problem. Log into the machine you dial into, then rlogin wherever you need to go. You will need a username and password on the remote machine.
You could probably create a login on the machine with the modem that uses rlogin as the login shell, but you would have to play with it to figure out how to make it work.
If what you really need is to dial in and be on your network without logging into a unix box, you will need a device called a terminal server. (You will still have to log into that.) There are many on the market, both as standalone devices and as options on routers (such as some Cisco models).