The behaviour of the module Unicom360.Webhook.Plug calls with each verified event.

defmodule MyApp.WhatsAppEvents do
  @behaviour Unicom360.Webhook.Handler

  @impl true
  def handle_event(%Unicom360.Webhook.MessageReceivedEvent{data: message}, _conn) do
    MyApp.Inbox.store(message.from, message.text)
    :ok
  end

  def handle_event(_event, _conn), do: :ok
end

Return :ok to answer 200. Anything else answers 500, and the gateway delivers the event again later. Answer quickly (hand slow work to a job queue), and drop repeats by the event's delivery_id.