There are things that the Avaya supplier (I don't care if they are authorized or not) did right and wrong. I cannot comment upon the Vodavi, it has been years since I looked at that manufacturer.

As previously stated by others the VM system is inadequate, HOWEVER they did add in the protection for the AC Line and the Telephone Lines, which the Vodavi supplier did not and should have. I myself only use sine wave UPSs for surge suppression. Both vendors missed some very important items and you need to clear them up.

I myself never take advice from those who know very little on the subject, a contractor in this case. A contractor builds a building and then is rarely seen, you will have visits much more often from your communications (voice & data) suppliers. I always ask my customers to involve me in the planning process of the building or remodel not after the fact.

My recommendation is to go back to both prospects with all the input you have received here today and see what relationship(s) you can build before you buy.