Rather than hardcoding the “Mix.Tasks.Compile.Elixir” function, you could just inject it in the init function on the GenServer. Then for testing you can just inject a function that sends the parameters back to the test for comparison.

defmodule Demo do
  def init([notify]) do
    {:ok, notify}
  end

  def terminate(reason, notify),
    do: notify.(:terminate, reason, self())

  def handle_call(request, {pid, _}, notify) do
    notify.(:call, request, pid)
    {:reply, :ok, notify}
  end

  def handle_cast(request, notify) do
    notify.(:cast, request, self())
    {:noreply, notify}
  end

  def handle_info(msg, notify) do
    notify.(:info, msg, self())
    {:noreply, notify}
  end

  def default_fun(t,r,p),
    do: IO.puts "#{t} #{inspect r} #{inspect p}"

end

run = fn demo, wait ->
  Process.sleep wait
  GenServer.call demo, {:hello,"there"}
  Process.sleep wait
  GenServer.cast demo, "For You Information"
  Process.sleep wait
  Kernel.send demo, 42
  Process.sleep wait
  GenServer.stop demo
  Process.sleep wait
end

fetch = fn wait ->
  receive do
    msg ->
      IO.inspect msg
  after
    wait ->
      :ok
  end
end

fun = &Demo.default_fun/3
{:ok, demo} = GenServer.start_link Demo, [fun]
run.(demo, 200)

dest = self()
test_fun = fn (t, r, p) -> Kernel.send dest, {t,r,p} end
{:ok, test} = GenServer.start_link Demo, [test_fun]
run.(test, 10)
fetch.(10)
fetch.(10)
fetch.(10)
fetch.(10)
{:message_queue_len, count} = Process.info self(), :message_queue_len
IO.puts "Message queue empty: #{count == 0}"

$ elixir demo.exs
call {:hello, "there"} #PID<0.80.0>
cast "For You Information" #PID<0.86.0>
info 42 #PID<0.86.0>
terminate :normal #PID<0.86.0>
{:call, {:hello, "there"}, #PID<0.80.0>}
{:cast, "For You Information", #PID<0.88.0>}
{:info, 42, #PID<0.88.0>}
{:terminate, :normal, #PID<0.88.0>}
Message queue empty: true
$

An excerpt how that might be used in ExUnit:

defmodule PhoneSupTest do
  use ExUnit.Case

  # ... setup and stuff ...

  # function that returns the function to be injected
  def notify_callback do
    pid = self()
    fn (reason, other, ms) ->
      Kernel.send pid, {reason, other, ms} # send tuple back to test process
      :ok
    end
  end


  test "PhoneSup - demo" do
    notify = notify_callback() # 1. Create function to inject
    ms1 = 1
    ms2 = 2

    {:ok, p1} = PhoneSup.attach_phone ms1, notify # 2. inject function
    {:ok, p2} = PhoneSup.attach_phone ms2, notify # 2. inject function

    assert (PhoneFsm.action {:outbound, ms1}, p2) == :ok # 3. test something
    assert_receive {:outbound, ^ms1, ^ms2}               # 4. verify contents
    assert_receive {:inbound, ^p2, ^ms1}                 #    of generated
                                                         #    messages

    # ... more assertions, etc ...

  end
end