Start Phoenix app with cowboy server on different port

To run phoenix app on a custom port other than port 4000, tweak your config file for respective environment(dev/prod/test) to the following:

config :my_app, MyApp.Endpoint,
  http: [port: {:system, "PORT"}],

Then from the terminal:

$ PORT=4001 mix phoenix.server
$ PORT=4002 mix phoenix.server

Enjoy.!