Don't use telnet. It's unencrypted and your password is sent in plain text. User SSH instead. Open port 22 on your router/firewall and redirect it to your Debian box. Then connect in using either SSH from a Unix box, or PuTTY from a Windows box
I use "telnet" as a simple way to test if bitcoin is alive from outside. I don't really want to manage bitcoin with "telnet". The goal is to use a real bitcoin client in the future.