KuJaX, like I said in my original post, check for a local listing in the Installers' tab at the top of the page.
Obviously you are a computer oriented individual, you have worked in a corporate environment (IVR, server, Avaya G10), and you want the bells and whistles of a large system without the cost.
Identify a good local dealer, let him educate you, bond with him, and as you grow you won't make large dollar investments without gaining the experience and expertise of a telecom professional.
(In English, this is NOT a DIY project)
Your telephone system is the gateway to your business for your customers. Professional or jack-leg? What will your customers think when they call?
I have a saying, when your phones aren't ringing you aren't making money.
Good Luck.
