There are MANY vendors out there for VoIP.
I reccommend going to the “Network Computing Website” and look at an article called “RFP: VoIP Invasion: Are you ready for it?”
It shows the RFP responses to imaginary company created by Network Computing, and analyzes Alcatel, Cisco, Avaya, and Nortel.
Also look online for articles from the recent “Voice Con 2001” it explores the solutions of at least 1/2 dozen vendors, and gives you some great insight into the differences from merely Voice over IP, and what a converged network are.
Other companies you want to look at include: Altigen, Clarent, Lucent, Mitel, Nec, Shoreline, Siemens, Sphere, and vertical.
There are others as well, (i.e. Fujitsu, 3Com, etc.) but they aren’t really being considered as heavy contendors in the market right now as far as I’m aware.
You mentioned that you are very happy with the NEC 2400 right now. There product is a modified 2400 (IP 2400) using software 8400, rev. 9.04.10
You may be able to have your exisiting 2400 upgraded to meet he specs of their IP one if you end up choosing them or trying them as a solution or test pilot.
Hope this helps. If you have futher questions, let me know.