Sometimes, only one side of the firewall can launch telnet sessions into the other side; however, some means of communication is possible (typically, through e-mail). Piercing the firewall is still possible, by triggering with whatever messaging capability is available a telnet connection from the ``right'' side of the firewall to the other.
fwprc includes code to trigger such connections
from a PGP-authentified mail;
all you need is add fwprc as a procmail filter
to messages using the protocol.
Note however, that if you are to launch pppd with appropriate priviledges,
you might need create your own suid wrapper to become root.
Instructions enclosed.
If you are firewalled, your mail may as well be in a central server that doesn't do procmail filtering or allow external telnet. No problem! You can use fetchmail to get mail to your client linux system, and add a cron-job to automatically poll for mail every 1-5 minutes, which will forward mail to a local address through sendmail, which itself will have been configured to use procmail for delivery. Too frequent a poll won't be nice to either the server or your host. Too unfrequent a poll means you'll have to wait before the message gets read and the reverse connection gets established. I use two-minute poll frequency.