Telnet is the command which we can easily use to check the service status of the remote system.
Ex if we can telnet to vCenter server from our desktop using port 443, we can say that web services of the vCenter is up
But when we use vCenter shell or ESXi shell, we cannot use telnet command.
As an alternative option, we can use curl command. Please refer the below screen capture
curl -v telent://vcsa-01a:443