telnet is usually used on port 23.
Port 23 is no weaker than any other port.
A hacker's ability to connect to a computer on any port is dependent on software listening on that port for connections and accepting connections. Then the hacker's ability is limited by what that software allows him to do. If a port is "open", i.e. not directly blocked by a firewall, then it's still effectively closed if there is nothing listening on that port.
So if you have no telnet server running on the computer in the other room, and no other software listening on port 23, then even if you have no firewall you won't be able to connect to that computer.