Non-notified centralized operator in private telecommunication network

A method for transmitting calls between interconnected PBX systems within a private telephone network is disclosed. An initial call connection request is generated at an originating PBX system to a destination PBX system. The call connection request includes an indication of a specific destination number which may answer a call to the exclusion of any other destination number. If the specific destination number may not answer the call, the originating PBX system is notified of the call connection failure and the originating PBX accesses a pre-generated listing of alternate call destinations. Call connection then proceeds to one of the alternate destinations.

BACKGROUND OF THE INVENTION 
1. Technical Field of the Invention 
The present invention relates to interconnection of a number of private 
branch exchange (PBX) systems, and more particularly, to communications 
between PBX systems that do not require day/night notifications to enable 
efficient call transfer from one PBX to another. 
2. Description of Related Art 
One manner in which PBX systems provide flexible and effective operator 
service is through the use of a centralized operator function and a 
day/night status notification function. These functionalities enable a 
satellite PBX to know the status of attached central PBXS. The day/night 
status notification function provides the satellite PBX with a 
notification of whether a central PBX is in the day or night operational 
status. 
During the day status, the central PBX is opened to receive calls and has 
human operators in place such that any call which is redirected from a 
satellite PBX may be completed. The night status indicates that the 
central PBX is presently not manned by a human operator. Thus, calls to 
numbers served by the satellite PBX shall not be redirected to the central 
PBX. The day/night status notification function within the central PBX 
notifies the satellite PBX of the status of the central PBX and the 
satellite PBX can refer to this information to select a new destination 
for directing calls of the satellite PBX while it remains in night status. 
The centralized operator function of a PBX system requires the support of 
the day/night status notification functions from the central PBXs in order 
to know the status of the various PBXs to which a satellite PBX is 
connected. In order to synchronize the day/night status data between the 
central and satellite PBXs, periodic notifying and updating are necessary. 
However, within a large private network, a central PBX may limit the 
number of satellite nodes which it notifies of its own day/night status in 
order to prevent adverse affects upon the private telecommunication 
network's overall performance. 
Also, no matter how often the notification is executed, there always exist 
a window between the time the status of a central PBX changes and the 
satellite PBX is notified. The situation may occur where a call is 
transferred from a satellite PBX to a central PBX whose status may have 
just changed. A method for eliminating the possibility of improper call 
connection attempts to PBX systems and for limiting housekeeping 
communications between PBX systems would be a very valuable improvement 
for PBX systems. 
SUMMARY OF THE INVENTION 
The present invention comprises a method and apparatus enabling 
transmission of calls between a number of interconnected PBX systems 
within a private telephone network. A function within a first PBX system 
generates a call connection request from the first PBX system to a 
specific destination number within a second PBX system. The call 
connection request includes an indication of the specific destination 
number which must answer the call such that the second PBX shall not 
attempt to forward the call to an alternate destination if the specific 
destination number cannot answer the call. 
If completion of the call to the specific destination number is not 
possible, the first PBX is notified of the incomplete call connection. The 
first PBX searches a listing containing alternate destination numbers for 
the originally transmitted call connection request and attempts to contact 
one of the alternate destination numbers in a similar manner. Routing of 
the call connection request is controlled solely by the call itself and 
may not be routed by the destination PBX or associated circuitry.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings, and more particularly to FIG. 1, there is 
illustrated a private telephone network 10. The private telephone network 
10 consists of a number of central PBX systems 15 which act as 
interconnecting nodes for a plurality of satellite PBX systems 20. The 
satellite PBX systems 20 may be connected to a single or multiple central 
PBX systems 15. As discussed previously, the ability of a satellite PBX 
system 20 to communicate with another satellite PBX system or central PBX 
system 15 is based upon a knowledge of whether a call request for operator 
service can be routed through its own or another central PBX system. This 
notification is presently provided by the day/night notifications between 
various PBX systems. The day/night status notifications are generated from 
a central PBX system 15 to a satellite PBX system 20, such that the 
satellite PBX system knows if the central PBX system has human operators 
in place. The notification may be implemented by either a query/respond 
system or a simple notification mechanism. 
Referring now to FIG. 2, there is illustrated the improved method of the 
present invention whereby an originating PBX system 25 which may comprise 
a satellite or central PBX system communicates with a destination PBX 
system 30 using a method that does not rely upon a day/night notification 
feature. In this method a non-notified centralized operator function 35 
residing within the originating PBX system 25 generates a Call Connection 
Request 40 to a destination number at a desired destination PBX system 30. 
The Call Connection Request 40 includes a restriction indicating a 
destination number which is the only number that can answer the call. If 
the call cannot be answered by the specified destination number, the 
destination PBX system 30 notifies the originating PBX system 25 that the 
call cannot be completed. The restriction to the specified destination 
number is a characteristic of the Call Connection Request and not of the 
destination number. 
Upon receipt of a rejection 45, the originating PBX system 25 utilizes the 
non-notified centralized operator function 35 to access a list 50 of 
alternate destination numbers. The list 50 of alternate destination 
numbers can be programmed per PBX, per call destination number, or in a 
combination thereof. The non-notified centralized operator function 35 
selects a next answering location from the pregenerated list 50 and 
initiates a new Call Connection Request 55 to a secondary destination PBX 
system 60. As before, the Call Connection Request 55 includes a 
restriction as to the destination number which may answer the call. If the 
call is answered at the secondary location, an acceptance 65 is 
transmitted to the originating PBX system 25 and call setup is completed. 
If the call cannot be answered at the secondary destination number, a 
rejection 70 is transmitted to the originating PBX system, and a next 
destination is selected from the list 50 as described previously. 
Referring now to FIG. 3, there is a flow diagram describing the method of 
the present invention. A Call Connection Request is initiated at step 80 
from the originating PBX system to the destination PBX system. Inquiry 
step 85 determines if the destination number associated with the Call 
Connection Request can answer the incoming call. If so, the call is 
connected at step 90. 
If the call cannot be answered by the destination number, the destination 
PBX system notifies the originating PBX system that the call cannot be 
completed at step 95. The originating PBX system then accesses the 
alternate number list at inquiry step 100 to determine whether there exist 
an alternate location to send the call connection request to. If so, the 
alternate number is returned and control passes back to step 80 to 
initiate a new Call Connection Request using the new number. If an 
alternate number does not exist, connection of the call fails at step 105. 
Thus, the non-notified centralized operator feature enables a satellite PBX 
system to redirect calls to an answering position and a non-notified PBX. 
If the call cannot be answered by the called party, the call will be 
rejected. The satellite (originating) PBX can then attempt to forward the 
call to the next choice of preprogrammed answering positions. 
Although a preferred embodiment of the method and apparatus of the present 
invention has been illustrated in the accompanying Drawings and described 
in the foregoing Detailed Description, it is understood that the invention 
is not limited to the embodiment disclosed, but is capable of numerous 
rearrangements, modifications, and substitutions without departing from 
the spirit of the invention as set forth and defined by the following 
claims.