This is an Illustration of a Private cloud
This can be with multiple sites using VPN or MPLS.  The main headquarters houses the Server which can be virtualized on VMWare (recomended) or a Standard Avaya Dell Server.  The Server also has the capabilities or housing a secondary server at a remote location.  There is also the ability to add a gateway to support both Digital and Analog trunks along with the ability for Digital and Analog end points as well.  Optional components include Fax servers, Paging equipment and a Call Center Server to name a few.