I have an Avaya PBX with Avaya 1616 IP Phones
during the last 2 month 6 phones had the same problem: the display lights died and the numbers are not visible
all the phones are under 3yr (bought them all together)
I cannot imagine that it is just bad luck or a coincidence and trying to look for a cause.
I couldn't find any documentation on this problem and Google didn't find me any useful links.