Please let him phone me back when he comes back.
从句子上理解为:请让他回来了之后给我回电话,回电话的人应该是那个还没回来的人。
please call me as soon as he comes back.
please ask him give me a call until he coming back.
please ask him to call me back when he is back.
please call me when he is back!
不要搞得那么复杂嘛!