Unfortunately, I can't give you as specific an answer as you might desire. However, there are a few things that you can do now to make sure that you have the best solution for your environment.
As you appear to have a firm understanding of your technical specifications and needs, I suggest that you create and advertise the release of an RFI. This is a very similar document to an RFP, except that it does not contain the obligation to select a bidder (especially given your nature as a state government department). The RFI should be detailed enough to solicit information from prospective vendors to allow you to make an informed choice, and your specs are a great place to start.
You also have at your disposal many different online groups that are populated with a variety of procurement professionals across many different disciplines. Places such as the National Association of Contract Managers, Institute for Supply Management and Caucus all have members-only discussion groups where members can ask for help from people who have already charted the waters. If you're not a member, you can usually find a member in your procurement, purchasing or contracts department.
Additionally, there are several specialized companies such as G2, Inc. (www.g2inc.com) who focus their business on telecommunications consulting and negotiations. If you combine that with information you can obtain from Garter (www.gartner.com), you should be able to get your hands around the various players in the industry. If all else fails, contact your Cisco and Avaya representatives and see if they will tell you who else is in this space.
This was first published in April 2005