There are other vendors out there, Cisco, Mitel, Shortel, etc.... We also sell Panasonic systems. Panasonic fills a gap/price point for the businesses that wants to rely on us to manage their phone system, and get their foot in the door with a phone system. The Panasonic from an administration side can be rough on a system administrator trying to do their own MAC work. Cisco and the others all have their benefits, and drawbacks. Each have their own pricing and feature models, so it is really a wash in a matter of speaking until you get into the realm of support, licensing, and integrating with 3rd party vendors.
If you do need to rely on a 3rd party vendor to integrate with your AVAYA solution, if the vendor is a Dev Connect partner you know it will work. Avaya spends weeks with vendors testing and creating documentation on how to integrate a multitude of solutions with their partners. Other competitors are lacking in this department.