|I wouldn't recommend telnet; it is inherently insecure. You should look into ssh instead.|
To answer your questions:
1. The remote computer needs to run a telnet server. By default few computers will run a telnet server.
2. Depends on the firewall on the remote LAN; most will block incoming connections using the telnet protocol.
3. Not sure what you mean by "flex on telnet". But to make any sort of remote connection you obviously need a working Internet connection.