How can I see what port a phoenix app in production is actually trying to use?

I am trying to debug the server. I believe that I have specified port 4000 in the configs. The server starts, but does not accept connections on port 4000. So in trying to debug this, one thought is that the port specification in the config is not correct. I was hoping to find a debug or verbose option that would show what port the server is attempting to bind to, so that I could verify that it is, indeed, port 4000. For example if the debug info told me that the server was trying to bind to a different port, then that would tell me that I have a problem with my port config.
Does this thought process make sense? Does anyone have a different approach to suggest?