How to install a telnet client?

apt-get install telnet

This is a valid question BTW. Do not install the telnet server, ever. Also the telnet client is not included by default in many distros. For instance it is very common in docker-land to face the need of installing several tools that are taken otherwise for granted. For example, Official OS docker images most likely will be shipped with a shell and its built-ins. Telnet will not be shipped in any of those.

A lot of docker images don't come with standard tools like vim, ping & telnet. You may need to install them explicitly.

sudo apt-get update && apt-get install telnet

You shouldn't need to install any packages; a telnet client should come standard with Ubuntu. Just open a terminal window (Alt + Ctrl + T) and use the telnet command (for help, type man telnet)