Page 178 describing the test send_sms_message/1. We are missing to mention that the test.exs should import the dev secrets

import_config "./dev.secret.exs"

If not we get into the error:

test/phone_app/conversations/conversations_test.exs:8
     ** (KeyError) key :number not found in: []

Which comes from not getting any value on (lib/phone_app/conversations/conversations.ex)

  def your_number do
    twilio_config = Application.get_env(:phone_app, :twilio, [])
    Keyword.fetch!(twilio_config, :number)
  end