Usually, you will find that when talking about PBXes, they are classified in Class 4 and Class 5. So, your next question is: What is the difference between a Class 4 and a Class 5 PBX? I will try to explain as clearly as possible.
Class 5
A Class 5 PBX deals with end-users. It provides final-user services such as voice mail, Conference rooms, IVR and many other kinds of services that a phone user may need. This kind of switch deals with user interaction (SIP endpoints such as Soft/Hard phones). So, in general terms, you can state that the principal client of a Class 5 PBX is the end-user.
It is very common to find them in the retail market.
Class 4
Class 4 PBX is a little different. They don't deal with end-users, but instead, they deal with other switches. If you are familiar with networking, you can see a Class 4 switch as a router that is connected to other routers. Class 4 ones do not offer conference rooms, IVR or any other endpoint item; authentication is usually tied to IP rather than registration.
You can find this kind of switch on the wholesale market.
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