That depends on many of things. Are you going to be using one Voice mail system or will you use 2?
Are all of the 2000 users in 2 locations or spread out?
If you are going to be using one system then it comes down to port capacity and network latency.
Each MAS will only handle 2 port card so if you are using T-1 QSIG that is a total of 46 ports per MAS.(If you are using Avaya Servers)
Then you have the question of N+1 so that if one goes off line or you need to load patched you will have at least one other MAS to take calls
Who came up with the current design ?
The pros and cons really depend on what you are ultimately looking for as the end result.
What system are you using now. Have you done a pilot test on MM with Exchange? If you use 2 systems you may also lose some features that you rely on today.
If you need more input let me know. All we do is messaging and related products.
Ken Means
"I find that the harder I work, the more luck I seem to have."
- Thomas Jefferson (1743-1826)