The biggest fault that Cisco has (IMO) is that it requires not one, not two, not three, but a virtual FARM of servers for each of it's services.
An Avaya system might have at best 5 or so servers, all of which are under maintenance with Avaya. No such thing with Cisco. You buy the servers, and maintain them yourselves.
The last system I was looking at for another company would have required 28 servers in a non-duplicated environment, and 42 in a high availability build.
Servers don't last very long. How long is usually dependent on what it's doing, but how many of us would keep a server longer than, say 5 years? Would you want to replace 28 or more servers every 5 years?
The more services you can put into hardware, the better off you are. And frankly, the Avaya product does that very well. Sadly, they're moving away from this strategy with systems like Modular Messaging. But as long as the larger system keeps a minimum of servers in it's architecture, I'll still prefer Avaya.
Carpe dialem! (Seize the line!)