As I understand it Levinas argues that the phenomenological conception of the other--ie., that of Sartre, Merleau-Ponty and Husserl---actually deprives the other of exactly that which would constitute their alterity. According to this interpretation, phenomenology almost invariably describes the other along the lines of what subjectivity knows of it (or at least thinks it knows).

If you recall, the phenomenological reduction is an attempt to bracket out the outside world, and to restrict itself to a description of the contents of consciousness. As a consequence of this theoretical starting point, it would seem that alterity can only be analysed according to how it appears to consciousness, and is hence defined in terms of what it is for the self. For Levinas, on the contrary, the other is precisely the opposite to this, being primarily that which resists knowledge as well as every attempt to thematise or capture that alterity.

John Wild in his 'Introduction' to Levinas' Totality and Infinity spells out the phenomenology of the other in terms of the meeting the other for the first time in his strangeness face to face. Wild says:

I see his countenance before me nude and bare. He is present in the flesh. But as Levinas points out in his revealing descriptions, there is also a sense of distance and even of absence in his questioning glance. He is far from me and other than myself, a stranger, and I cannot be sure of what this strangeness may conceal. Hence the need to show friendly intent which bought forth the earliest forms of introduction and greeting. (p.13)

Wild adds that this is but the beginning of the phenomenology of the other. He adds:

Even if he comes with no ill will, he remains a stranger inhabiting an alien world of his own. Of course, I may simply treat him as a different version of myself, or, if I have the power, place him under my categories and use him for my purposes. But this means reducing him to what he is not. How can I coexist with him and still leave his otherness intact?