The second is correct in this sentence; the first is incorrect. "Had left" must be in relationship to some other time, normally past. Because you are discussiong an event in relationship to the present, the "have" form is used. So Katherine's example for the "had" form ( "I could not be reached on my mobile last Tuesday because I had left it at home.") is correct because it is in reference to past event or time, ie "last Tuesday."