Telnet is more often available in older/properly configured windows systems. Curl is nix and doesn’t have the history of availability of telnet. Good to know both.
Wonder whether ssh -verbose could work in a pinch? Haven’t tried it for non-ssh coms.
The whole point of course is not every protocol nor service provides a minimalistic/widespread way of testing for basic connectivity.
If you just want to test if something is listening on a port, no need to specify the "telnet" protocol - using curl normally with the verbose option will still tell you whether the port is open, though it'll error out later down the path when it doesn't get the expected HTTP response if probing a non-HTTP port.
