Don't compare Avaya with Mitel, you'll break my heart.....
In the Avaya CM environment when High Availabilty is needed then you are drowning in the added servers, for each service a minimum of two servers is needed.
They could better walk the way Kyle suggest, that is a whole lot more reliable as adding single mirror servers running for no use ( normally ) and consuming a lot of expensive energy. A well setup VMware solution will reduce down time to a absolute minimum (if any) and energy consuption will be far less.
If it ain't dutch it ain't much