The Avaya voice mail that was quoted only offers 4 mailboxes. It will not handle the anticipated growth of your own practice let alone a 2nd entity. You will want to have the Avaya dealer upgrade his quote to include the current version of the more robust Partner Messaging which will add a decent chunk of change to the price. If you truly expect to have two operations you want to look at a system that offers "tenant services" capability. This will allow each entity to have separate "zero out" destinations and music on hold. The Vodavi brand of equipment is just fine as far as quality is concerned and if the company selling them is highly recommended and the warranty appears better then as long as the system does what you want it seems like the decision should be fairly simple. You should ask for some references from both companies - ideally you will find someone else you know and trust on their reference list.