Avaya S8720 controllers running CM 4.0.1. Pure IP.
Cisco wasn't the dominant system, and it is installed in a relatively small Trading area. Avaya replaced (transition not yet fully completed) an older NEC system that is still connected to the Cisco via a T1 (all trunking is on the NEC except for some ringdown lines that are directly on the Cisco).
Since the NEC is scheduled to disappear, and to improve reliability and give some breathing space for Trading to decide what they ultimately want to have, the Avaya is to be connected to the Cisco instead, hence the original posting.
We can't run any quick tests at this time, although a test is being discussed. Any changes or experiments with the systems have to go through a vetting process - another story.
H.323 alone may support all the required functions, but if we determine that voice messaging through the Avaya is now needed, then it's likely that QSIG will be required, and without any hard evidence that it can be done between Avaya and Cisco over H.323, then it will end up over a T1.
As for why Avaya and not Cisco, or an upgrade to the NEC - it's a long story. There is no perfect system, and as a time goes by relative strengths and weaknesses change as well, so yesterday's decision can be today's headache.