Folks, if you want to try a true IP-PBX, don't spend thousands, just go to Berkeke.com and try their OnDO PBX. You can get a free trial version that requires reset every 3 hours, or you can just buy the SOHO version for a mere $300 USD. They even give you instructions on how to setup their IP-PBX with current VoIP vendors. All it takes is a small PC (I have used an old PIV) with only 512 Megs RAM. You can download it from the net and be running the IP-PBX within an hour. The IP-PBX also comes with a SIP Server as well. It runs on WindowsXP or Linux.
I don't work for Berkeke or have any financial relationship with them; however, I have used their products with great success with my clients. They even have a Java SDK interface for developer(s) and those that want to enhance or modify the IP-PBX features. Very cool.
Try for free before spending anything ;-) Have a good one, gents.
Upstate NY / Alaska