You can't telnet to the outside of a PIX (and that's a good thing, too big a security hole). You can only to telnet to the other interfaces (inside/dmz). You can however SSH to the PIX on the outside interface (any interface for that matter).
See link on which version you have and how to enable SSH (add hostname, domain name, telnet password, SSH address, generate a key) on the PIX: http://www.cisco.com/warp/public/110/pix_command_ref.shtml
Limit the IPs that can SSH into the PIX, and look into using AAA with it as well to maximize security.
Hope it helps.
Steve