I work in the Com system biz.
I install Siemens, Avaya, and a IP/Sip product called Vonexus. Vonexus runs on a Windows 2003 server and works over a array of SIP based phones. The main ones are Polycom phones. You can span and scale the sytem across the country by sharing the same network connection up to 15,000 users. Vonexus has the most feature rich envirenoment allowing you to do just about anything you want.
----Unified msg, follow me, work at home client, make intelligent decisions based on caller ID, PC clients letting you see users logged in or on the phone, record your calls that go to your mail box or record all calls all the time live and store them to a database server. You can record calls based on criteria that fits an agenda, like certain extensions, or between a certain time, or calls that originate from a cetain CID. You can listen in on calls, and get detailed call detail reports in more ways that are too numerous to list. The system can do txt to voice or voice to txt. It can screen your calls when forwarded to your cell phone to announce who is calling, then decide whether you want to answer or send them to voicemail. Call center managers can have a supervisor client that gives them live istant statistics of ACD users as it happends. No more waiting to run reports to figure out what lines or users need more assitance.
The price and features of Vonexus make it extremely cost effective and a no brainer. We have installed these systems all across the US. Since it is an Indianapolis based company many state munincipalities across Indiana have installed Vonexus. The company that produces this system is called Interactive Inteligence Inc. You can check them out at
As far as other systems, the smaller Avaya system, the IP Office, is also an excellent system that is a hybrid between traditional digital phones and IP based. They have PC clients that can be used also to act as a busy lamp and to record personal calls or control the phone.
Siemens 4000 is also a hybrid and is a very stable system and can do many of the basic IP features if needed.
By far Vonexus gives way more for the money.