I won't patronise you by asking if you have checked the Install/Admin Guide for this phone, but what I will say is that as I am not experienced with this type iof issue, my first port of call was to do that myself, as well as some general research. What strikes me as rather odd, as so far, although I can find documents confirming this model is supported by Avaya Aura, CS, CM etc. I have not been able to find the minimum versions for compatiility, phone firmware required etc. Is that not rather odd....? (for any PBX vendor, not jsut Avaya...)
It is difficult to comment on Prognosis showing the phone as 'up', when we rely (mainly) on information we get from CM etc. So I think the basic regisration/config problem is one for Avaya Support, before IR can comment on this. Also, I would point out the Prognosis does not (currently!) "manage" phones....
I'm apologise that this is not much help to you, but I suspect my colleagues with more experience with Avaya would likely make the same suggestion.