Mitel is another one. They're a smaller company with some great products. I'd probably go with them before I'd go with Avaya. Our current evaluation of Nortel, Avaya, Mitel, and Cisco is coming down to basically Mitel and Cisco.
Enterprise and for me is a pain to manage so many servers yes the scripting language is slick, but remember avaya's vectors are done in the call processing not needing me to run 10+ server for 700 agents and allowing to run some pretty cool disaster recovery options throught the world without replicating server farms. In addition I run avaya's CMS reporting server "which is kick butt" and interaction center software which is light years ahead in the Multi-channel call center world..And just an fyi pre-routing is now "old school"in the call center world (being ripped out left and right).
Also, got love all the old lucent folks working at cisco thinking nothing has been updated with the avaya software in the last 5 years:) makes me laugh.
My cisco AM keeps on telling me that ccm is going to have a linux version soon and some new call center software, just what i need to be a beta tester for cisco hahaha...