The short answer is: you can’t use telnet. But you can use alternatives, obviously.
For instance, to troubleshoot some iSCSI connectivity problems, you would be used to doing something as this.
~ # telnet 10.0.2.3 3260 -ash: telnet: not found
Instead, you can use netcat to test the connectivity.
~ # nc -z 10.0.2.3 3260 Connection to 10.0.2.3 3260 port [tcp/*] succeeded!
If you were to test this on a closed TCP port, the error would look like this.
~ # nc -z 10.0.2.3 32600 (empty response)