Probably the Partner R7, the first major change when Avaya took over from Lucent. They took the 3x8 R6 and made a 5x9 and of course anything they touched turned to crap. The R7 had all kinds of bugs, so much so that I never sold one, sold the R6 until they came out with the R8.
Did your customer just recently put the AA on auto day/night schedule or has it been that way working properly and within the last 3 days started doing this? What messaging system?
-Hal