Patent Publication Number: US-8116435-B2

Title: Emergency call processing apparatus and method in private switching system

Description:
CLAIM OF PRIORITY 
     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C.§119 from an application for APPARATUS AND METHOD FOR PROCESSING EMERGENCY CALL IN PRIVATE SWITCHING SYSTEM earlier filed in the Korean Intellectual Property Office on the 20 th  of Jan. 2006 and there duly assigned Ser. No. 10-2006-0006489. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to an emergency call processing apparatus and method in a private switching system and, more particularly, an apparatus and method by means of which an extension subscriber of a private branch exchange can make an emergency call even if all lines are busy, as well as originate an emergency call with an extension terminal even if it is locked. 
     2. Description of the Related Art 
     In general, subscribers of a private switching system are graded in call origination, and thus sometimes they are screened from calling to specific telephone numbers. Then, dialing even to an emergency telephone number may be screened also, and thus sometimes an emergency call cannot be processed. 
     For example, if the calling function of an extension terminal is set in a locking mode, when a user dials a general telephone number or an emergency telephone number, an outgoing call is not transmitted to the corresponding number. Then, the user has to input a preset password, and only if the inputted password is validated, the calling function can be moved from the locking mode so as to transmit an outgoing call intended by the user. 
     Furthermore, if all lines connected to the private branch exchange are busy, the user of the extension terminal cannot have a call until a line becomes available. 
     In a private switching system adopting such technology, if an available line does not exist or a terminal is set in a locking mode, an extension subscriber cannot make a call, even in the case of an emergency. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide an emergency call processing apparatus and method in a private switching system, by means of which an extension subscriber of a private branch exchange can make an emergency call even if all lines are busy, as well as originate an emergency call with an extension terminal even if it is locked. 
     That is, according to an object of the invention, when an emergency telephone number is inputted from an extension subscriber terminal, even if all lines are busy, at least one busy line is intercepted so that an emergency outgoing call can be transmitted via the intercepted line. When the extension terminal is in a locking mode, if an inputted telephone number is equivalent to or matches an emergency telephone number, the locking mode is disabled so that a corresponding emergency outgoing call can be processed. 
     According to an aspect of the invention, an emergency call processing method in a private switching system comprises the steps of: upon receiving a telephone number for call origination from a user in a locking mode, determining whether the inputted telephone number is equivalent to or matches at least one preset emergency telephone number; and, if the inputted telephone number is equivalent to or matches at least one preset emergency telephone number, disabling the locking mode and transmitting an outgoing call corresponding to the inputted telephone number to a private exchange branch. 
     The determining step preferably comprises: if the telephone number inputted by the user is not equivalent to or does not match any one of the emergency telephone numbers, displaying digit values of the inputted telephone number by masking; comparing the digit values of the inputted telephone number sequentially to digit values of a password for disabling the locking mode; and, if the inputted telephone number matches the password, disabling the locking mode and transmitting the digit values of the inputted telephone number to the private branch exchange. 
     The displaying step preferably comprises: if the inputted telephone number is equivalent to or matches at least one of the emergency telephone numbers, displaying the digit values of the inputted telephone number without masking. 
     According to another aspect of the invention, an emergency call processing method in a private switching system comprises the steps of: when digit values of a telephone number for call origination received from a private subscriber terminal match digit values of at least one preset emergency telephone number, determining whether an available line exists at present; and, if an available line does not exist at present, intercepting at least one occupied line according to a preset priority, allocating the intercepted line for emergency call origination, and transmitting an emergency outgoing call via the allocated line. 
     The preset priority in the step of intercepting at least one occupied line preferably comprises at least one of priority according to line-occupied time and priority according to line number set for emergency call origination. 
     According to still another aspect of the invention, an emergency call processing method in a private switching system comprises the steps of: at a subscriber terminal, upon receiving a telephone number for call origination from a user in a locking mode, determining whether the inputted telephone number is equivalent to or matches at least one of preset emergency telephone number; if the inputted telephone number is equivalent to or matches at least one of the preset emergency telephone numbers, at the subscriber terminal, disabling the locking mode and transmitting digit values of the inputted telephone number to a private exchange branch; at the private exchange branch, upon receiving the digit values from the subscriber terminal, determining whether a line available for emergency call origination corresponding to the received digit values exists; and, if the available line does not exist at present, at the private exchange branch, intercepting at least one occupied line according to preset priority, allocating the intercepted line for emergency call origination, and transmitting an emergency outgoing call via the allocated line. 
     The comparing step preferably comprises: when the telephone number inputted by the user is not equivalent to or does not match any one of the emergency telephone numbers, displaying digit values of the inputted telephone number by masking; comparing the digit values of the inputted telephone number sequentially with digit values of a password for disabling the locking mode; and, if the inputted telephone number matches the password, disabling the locking mode and transmitting the digit values of the inputted telephone number to the private branch exchange. 
     The emergency call processing method preferably further comprises synchronizing and registering the emergency telephone numbers between the subscriber terminal and the private branch exchange when initially or additionally registering or deleting each of the emergency telephone numbers. 
     The step of synchronizing and registering the emergency telephone numbers preferably comprises: at the subscriber terminal, upon receiving a request for emergency telephone number registration or deletion from the user via a user interface (UI) of the subscriber terminal, updating its own database according to information on a requested emergency telephone number, and transmitting emergency telephone number update information to the private branch exchange; and, at the private branch exchange, updating its own emergency telephone number database according to the emergency telephone number update information transmitted from the subscriber terminal, and transmitting the updated information to a second terminal so as to synchronize with the second terminal. 
     The step of synchronizing and registering the emergency telephone numbers preferably comprises: at the private branch exchange, upon receiving a request for emergency telephone number registration or deletion from the user, updating its own database according to information on the requested emergency telephone number, and transmitting emergency telephone number update information to the subscriber terminal so as to synchronize with the subscriber terminal. 
     According to yet another aspect of the invention, an emergency call processing method in a private switching system comprises the steps of: at a subscriber terminal, upon receiving a telephone number for call origination from a user, determining whether a locking mode is set; at the subscriber terminal, if the locking mode is not set, transmitting digit values of the inputted telephone number to a private branch exchange; at the private branch exchange, upon receiving the digit values from the subscriber terminal, comparing the received digit values with digit values corresponding to emergency telephone numbers previously stored; at the private branch exchange, if the received digit values match the digit values corresponding to at least one of the emergency telephone numbers previously stored, determining whether a line available for emergency call origination exists; and, at the private branch exchange, if the available line does not exist at present, intercepting at least one occupied line according to a preset priority, allocating the intercepted line for emergency call origination, and transmitting an emergency outgoing call via the allocated emergency call line. 
     According to another aspect of the invention, an emergency call processing apparatus in a private switching system comprises a call processing unit which, if a telephone number for call origination inputted by a user in a locking mode is equivalent to or matches at least one preset emergency telephone number, disables the locking mode and transmits an outgoing call corresponding to the inputted telephone number to a private branch exchange. 
     The call-processing unit preferably comprises: a memory for storing, in a digital format, the emergency telephone numbers and a password for disabling the locking mode; a display unit for displaying digit values of the inputted telephone number by masking; and a controller which, if the inputted telephone number from the user is not equivalent to or does not match any one of the emergency telephone numbers, masks the digit values of the inputted telephone number, controls the display unit to display the digit values of the inputted telephone number, and compares the digit values of the inputted telephone number sequentially with digit values of the password for disabling the locking mode, and which, if the inputted telephone number is equivalent to or matches the password, disables the locking mode and transmits the digit values of the inputted telephone number to the private branch exchange. 
     Preferably, if the telephone number inputted in the locking mode is equivalent to or matches at least one of the emergency telephone numbers, the controller controls the display unit to display the digit values of the inputted telephone number without masking. 
     According to another aspect of the invention, an emergency call processing apparatus in a private switching system comprises a call processing module which, if digit values of a telephone number for call origination received from a private subscriber terminal are equivalent to or match digit values of at least one preset emergency telephone number, determines whether an available line exists at present, and if the available line does not exist at present, intercepts at least one occupied line according to a preset priority, allocates the intercepted line for emergency call origination, and transmits an emergency outgoing call via the allocated line. 
     The call processing module preferably comprises a memory for storing information on the emergency telephone numbers and the preset priority for intercepting at least one of the occupied lines. 
     The priority information for intercepting at least one of the occupied lines preferably comprises at least one of priority information according to line-occupied time and priority information according to line number set for emergency call origination. 
     According to a further aspect of the invention, a private switching system comprises: a subscriber terminal which, if a telephone number for call origination inputted by a user in a locking mode is equivalent to or matches at least one preset telephone number, disables the locking mode and transmits digit values of the inputted telephone number; and a private branch exchange which, upon receiving the digit values for call origination from the subscriber terminal, determines whether an available line exists at present, and if the available line does not exist at present, intercepts at least one occupied line according to a preset priority, allocates the intercepted line for emergency call origination, and transmits an emergency outgoing call via the allocated line. 
     The subscriber terminal preferably comprises: a memory for storing, in a digital format, the emergency telephone numbers and password for disabling the locking mode; a display unit for displaying digit values corresponding to the inputted telephone number by masking; a controller which, if the inputted telephone number from the user is not equivalent to or does not match any one of the emergency telephone numbers, masks the digit values of the inputted telephone number and controls the display unit to display the digit values of the inputted telephone number, and which, if the inputted telephone number is equivalent to or matches the password, disables the locking mode and transmits the digit values of the inputted telephone number sequentially to the private branch exchange. 
     According to yet another aspect of the invention, a private switching system comprises: a subscriber terminal which, upon receiving a telephone number for call origination inputted by a user when a locking mode is not set, transmits digit values of the inputted telephone number; and a private branch exchange which, upon receiving the digit values from the subscriber terminal, if the received digit values are equivalent to or match digit values corresponding to at least one of the emergency telephone numbers previously stored, determines whether a line available for emergency call origination exists, and if the available line does not exist at present, intercepts at least one of occupied lines according to a preset priority, allocates the intercepted line for emergency call origination, and transmits an emergency outgoing call via the allocated emergency call line. 
     The priority information for intercepting at least one of the occupied lines preferably comprises at least one of priority information according to line-occupied time and priority information according to line number set for emergency call origination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
         FIG. 1  is a block diagram of an emergency call processing apparatus and network links thereof in a private branch exchange system according to the invention. 
         FIGS. 2A and 2B  are flowcharts illustrating an emergency call processing method in a private branch exchange system according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of a packet processing apparatus and method of the invention are shown. 
       FIG. 1  is a block diagram of an emergency call processing apparatus and network links thereof in a private branch exchange system according to the invention. 
     As shown in  FIG. 1 , the private switching system includes a plurality of subscriber terminals  100 , . . . ,  100   n -1 and  100   n , and a private branch exchange  200 . 
     Each of the subscriber terminals  100 , . . . ,  100   n -1 and  100   n  includes a digit input unit  110 , a controller  120 , a display unit  130 , a memory  140  and an interface  150 . 
     The private branch exchange  200  includes a subscriber interface processing module  210 , a call processing module  220  and a line-processing module  230 . The call processing module  220  includes a switch  221 , a controller  222  and a memory  223 . 
     With respect to the aforementioned operation, it will be described only in terms of emergency call processing functions of the invention, but not in terms of those functions, such as general signal processing, unrelated to the emergency call processing functions, in order to clarify the scope of protection of the invention. 
     When the subscriber terminal  100  is in a locking mode (also called “outgoing call screening mode”), the digit input unit  110  of the subscriber terminal  100  inputs digits into the controller  120 . The digits include password digits for disabling the locking mode and telephone number digits of a receiving terminal, such as an emergency telephone number in use for call origination. 
     The display unit  130  displays digit values introduced from the digit input unit  110  under the control of the controller  120 . In the locking mode of the subscriber terminal  100 , when displaying the digit values introduced through the digit input unit  110 , the display unit  130  acts to mask the digit values in order to prevent password leakage. 
     The memory  140  stores emergency telephone number information stored in the memory  223  of the call processing module  220  of the private switching system  200  and digit values of synchronized emergency telephone numbers, and also stores digit values of a password for disabling a locking mode in different memory areas. In an area of the memory  140 , information on setting and disabling the locking mode of the subscriber terminal  10  is stored. The locking mode setting or disabling can be set by “1” or “0” like a flag. 
     The controller  120 , upon receipt of digits via the digit input unit  120 , confirms a locking mode setting flag stored in the memory  140  to determine whether the subscriber terminal  100  is currently set to the locking mode. 
     If it is determined that the subscriber terminal  100  is not set to the locking mode, the controller  120  displays inputted digit values sequentially on the display unit  130 , and at the same time, transmits the digit values to the private branch exchange  200  via the interface  150  if all of the digit values are inputted. 
     However, if the subscriber terminal  100  is currently set to the locking mode, the controller  120  compares the inputted digit values to digit values corresponding to at least one emergency telephone number stored in the memory  140 . If the inputted digit values are equivalent to or match the digit values corresponding to the emergency telephone number, the controller  120  displays the inputted digit values via the display unit  130  without masking them. 
     If any of the inputted digit values are not equivalent to or do not match the digit values corresponding to the emergency telephone number, the controller  120  determines the input digits as those corresponding to a password, and masks the remaining digit values including those which have been displayed already without masking, so that all of the digit values are displayed via the display unit  130 . 
     Then, the controller  120  sequentially compares the inputted digit values to the digit values corresponding to the password stored in the memory  140 . If the inputted digit values match the digit values corresponding to the password, the controller  120  controls the locking mode setting flag in the memory  140  into a locking mode disabling flag so as to disable the locking mode, and then transmits subsequent inputted digit values to the private branch exchange via the interface  150 . 
     However, if the inputted digit values do not match the digit values corresponding to the password stored in the memory  140 , that is, input numbers are not identical to the password, the controller  120  instructs the display unit  130  to display a message requesting re-input of a password due to unmatched password input. 
     In the meantime, if all of the inputted digit values match digit values corresponding to one of emergency telephone numbers stored in the memory  140 , the controller  120  determines an emergency call origination, and then disables the locking mode set in the memory  140  and transmits the digit values of the emergency telephone number inputted via the digit input unit  110  to the private branch exchange  200  via the interface  150 . 
     After an emergency telephone call is originated in the locking mode and then call communication is terminated, when a call termination signal with respect to the emergency outgoing call is received from the private branch exchange  200 , the controller  120  converts the locking mode disabling flag into a locking mode setting flag so as to lock the subscriber terminal again. As a result, this relieves the user of the inconvenience of resetting the locking mode. 
     The subscriber interface processing module  210  of the private branch exchange  200  is interfaced with the subscriber terminals  100 , . . . ,  100   n -1 and  100   n  so as to interface-process signals transmitted between the private branch exchange  200  and the subscriber terminals. 
     The switch  221  of the call processing module  220  switches an outgoing call to the line-processing module  230 , which corresponds to the call telephone number of the subscriber terminal  100  received via the interface-processing module  210 . 
     The memory  223  of the call processing module  220  stores emergency telephone number information stored in the memory  140  of the subscriber terminals  100 , . . . ,  100   n -1 and  100   n , synchronized therewith. 
     When the subscriber terminal  100  is not in the locking mode, the controller  222  of the call processing module  220  sequentially compares the digit values corresponding to the emergency telephone number transmitted from the subscriber terminal  100  to the digit values corresponding to at least one emergency telephone number stored in the memory  223 . 
     If the digit values corresponding to the telephone number received from the subscriber terminal  100  match the digit values corresponding to one of the emergency telephone numbers stored in the memory  223 , the controller  222  checks the line processing module  230  to find any available line in existence at the time. 
     If an available line exists, the controller  222  allocates the available line to provide a switching control signal to the switch  221  so that an outgoing call for the emergency telephone number corresponding to the input digits transmitted from the subscriber terminal  100  is transmitted via the line processing module  230  to the Public Switched Telephone Network (PSTN). 
     However, if an available line does not exist, the controller  222  intercepts one of busy lines according to preset conditions, and controls the switch  221  to switch the intercepted line as a line for transmitting the outgoing call for the emergency telephone number received from the subscriber terminal  100 . Then, the originating signal for the emergency telephone number is transmitted via the line-processing module  230  to the PSTN. In this regard, the reference for intercepting one of the busy lines may be selectively set. For example, a line having the longest handling time may be intercepted based on counted handling times, or a line may be set for an emergency outgoing call so that, when the subscriber requests the emergency outgoing call with no available lines, the set line can be intercepted. 
     In the meantime, if the inputted digit values received from the subscriber terminal  100  do not match any of the emergency telephone numbers stored in the memory  223 , that is, the telephone number selected by the subscriber terminal  100  is not any of the emergency telephone numbers, the controller  222  transmits an outgoing call signal via the line processing module  230  or sends a busy tone signal to the subscriber terminal  100  based on the existence of available lines. 
     When the locking mode of the subscriber terminal  100  is disabled by the input of the emergency telephone number, if the digit values of the emergency telephone number are received from the subscriber terminal  100 , the controller  222  provides the inputted digit values to the line processing module  230  via the switch according to the existence of an available line so that the line processing module  230  processes an emergency outgoing call received from the subscriber terminal  100  via the PSTN. Upon completion of the emergency outgoing call communication according to emergency outgoing call processing, the controller  222  transmits an emergency outgoing call completion signal to the subscriber terminal  100  so that the subscriber terminal  100  can be set to the locking mode again. 
     As for the functions and operations of the aforementioned components, call receipt via the PSTN will not be described. 
     A more detailed description of the operation of the emergency call processing apparatus in the private switching system of the invention having the previously described structure will now be presented. 
     First, the memory  140  of the subscriber terminal  100  and the memory of the call processing module  220  of the private branch exchange  200  are designed to store at least one registered emergency telephone number, synchronized with each other, in order to enable emergency call processing of the invention. In this regard, operations for registering the emergency telephone number, and then synchronizing the emergency telephone number between the subscriber terminal  100  and the private branch exchange  200 , will be described briefly. 
     When a user inputs an emergency telephone number through a user interface (UI) of the subscriber terminal  100  in order to initially register or update it, the controller  120  sequentially stores information on the inputted emergency telephone number in the memory  140 , and then transmits the stored emergency telephone number information to the private branch exchange  200  via the interface  150 . 
     The controller  222  of the private branch exchange  200  sequentially stores the received emergency telephone number information in the memory  223 , and then broadcasts the emergency telephone number information to the extension terminals or subscriber terminals  100   n -1 and  100   n  via the subscriber interface processing module  210  so that the emergency telephone number information is synchronized with the subscriber terminals  100   n -1 and  100   n.    
     While the above description has been made only with regard to a registration operation by the subscriber terminals  100 , . . . ,  100   n -1 and  100   n , even if the emergency telephone number information is initially registered or updated by the private branch exchange  200 , it can be transmitted to the extension or subscriber terminals  100 , . . . ,  100   n -1 and  100   n  and thus synchronized therebetween. In this case, the emergency telephone number information synchronized between the memory  140  of the subscriber terminal  100  and the memory  220  of the call processing module  220  of the private branch exchange  200  may be stored with a memory format as shown in Table I below. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 No 
                 Digit 
                 Length 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 1 
                 119 
                 3 
               
               
                 2 
                 112 
                 3 
               
               
                 3 
                 2794301 
                 7 
               
               
                 . . . 
                 . . . 
                 . . . 
               
               
                 N 
                 None 
                 0 
               
               
                   
               
            
           
         
       
     
     As in Table I above, it should be understood that the user can selectively set the maximum number allocatable for emergency telephone number and their digits. 
     In this situation wherein the emergency telephone number information is synchronized between the extension or subscriber terminals  100 , . . . ,  100   n -1 and  100   n  and the private branch exchange  200 , when digit values corresponding to an outgoing telephone number are inputted via the digit input unit  110  of the subscriber terminal  100 , the controller  120  of the subscriber terminal  100  inspects a locking mode setting flag to determine whether a locking mode is set. 
     If the locking mode is not set, the controller  120  transmits the digit values corresponding to the outgoing telephone number inputted from the digit input unit  110  to the private branch exchange  200  via the interface  150 . 
     The subscriber interface-processing module  210  of the private branch exchange  200  provides the digit values transmitted from the subscriber terminal  100  to the controller  222  of the call processing module  220 . 
     The controller  200  of the call processing module  200  sequentially compares the digit values inputted from the subscriber interface processing module  210  to those corresponding to any of emergency telephone numbers stored in the memory  223  in order to determine whether the digit values from the subscriber interface processing module  210  correspond to any of the emergency telephone numbers. 
     If the digit values of the emergency telephone number transmitted from the subscriber terminal  100  match one of the emergency telephone numbers stored in the memory  223 , the controller  222  determines the telephone number received from the subscriber terminal  100  to be the emergency telephone number, and then checks lines of the line processing module  230  via the switch  221  to find any available lines for processing a present outgoing emergency call. 
     If an available line is found, the controller  222  controls the switch  221  to provide the received outgoing call to the line-processing module  230  via the available line. The line-processing module  230  generates a DTMF signal corresponding to digit values for the switched emergency outgoing call, and transmits it to the PSTN via the allocated (available) line. 
     However, if an available line does not exist at the time, the controller  222  provides a control signal to forcibly intercept one of the busy lines so that the line processing module  230  intercepts one line under the control of the controller  222 . Then, the controller  222  allocates the forcibly intercepted line for emergency call processing, and controls the switch  221  to switch the digit values for the emergency call to the line-processing module  230 . In this regard, the reference for intercepting one of the busy lines may be selectively set. For example, a line having the longest handling time may be intercepted based on counted handling times, or a line may be set for an emergency outgoing call so that, when the subscriber requests the emergency outgoing call with no available lines, the set line can be intercepted. 
     If the digit values corresponding to the called telephone number transmitted from the subscriber terminal  100  do not match those corresponding to one of the emergency telephone numbers stored in the memory  223 , that is, if the telephone number from the subscriber terminal  100  is not one of the emergency telephone numbers, the controller  222  of the call processing module  220  of the private branch exchange  200  checks the switch  221  and the line processing module  230  to find an available line. 
     If an available line exists, the controller  222  controls the switch  221  to provide a received outgoing call (i.e., the digit values for the called telephone number) to the line processing module  230 . The line processing module  230  generates a Dual Tone Multi-Frequency (DTMF) signal corresponding to the switched digit values for the outgoing call emergency, and transmits the DTMF signal to the PSTN via the allocated (available) line. 
     However, if an available line does not exist, the controller  222  transmits a busy tone signal to the subscriber terminal  100  via the subscriber interface-processing module  210 . 
     When digit values are inputted through the digit input unit  110  of the subscriber terminal  100 , if the subscriber terminal  100  is set to the locking mode, the controller  120  of the subscriber terminal  100  compares the inputted digit values to digit values corresponding to the emergency telephone number information stored in the memory  140  in order to determine whether the inputted digit values correspond to one of the emergency telephone numbers. 
     If the inputted digit values match digit values corresponding to one of the emergency telephone numbers, the controller  120  sequentially displays the inputted digit values on the display unit  130  of the subscriber terminal  100 . 
     If all of the inputted digit values do not match the digit values corresponding to the emergency telephone numbers stored in the memory  140 , that is, only some of the inputted digit values match some of the digit values corresponding to the emergency telephone numbers, all of the inputted digit values, including those displayed up to the present, are masked and then displayed on the display unit  130 . However, if a first one of the inputted digit values does not match any of first digit values of the emergency telephone numbers, the inputted digit values from the beginning are masked and displayed on the display unit  130 . 
     In this regard, the “masking” is conducted to prevent password leakage, based on the judgment that the inputted digit values are those corresponding to a password for disabling the locking mode rather than those corresponding to any of the emergency telephone numbers. 
     If all of the inputted digit values from the digit input unit  110  match the digit values corresponding to one of the emergency telephone numbers stored in the memory  140 , the controller  120  converts a locking mode setting flag in the memory  140  into a locking mode disabling flag, thereby disabling the locking mode, and then transmits the inputted digit values received from the digit input unit  110  to the private branch exchange  200  via the interface  150 . 
     However, if all of the inputted digit values do not correspond to any of the emergency telephone numbers and thus inputted in a masked and displayed state, the controller  120  compares the inputted digit values to the digit values corresponding to password(s) stored in the memory  140 . 
     If the comparison shows that the inputted digit values do not match those corresponding to a password, the controller  120  displays a message on the display unit  130 , requesting re-input of password. 
     However, if all the inputted digit values do match those corresponding to a password, the controller  120  disables the locking mode set in the memory  140 , and transmits digit values corresponding to a called telephone number, when inputted since then via the digit input unit  100 , to the private branch exchange  200 . 
     Accordingly, the controller  222  of the call processing module  220  of the private branch exchange  200  determines whether an available line exists in order to process an outgoing call for the called telephone number transmitted from the subscriber terminal  100 . If there is no available line, the controller  222  transmits a busy tone signal through the subscriber interface-processing module  210  to the subscriber terminal  100  which has sent the outgoing call. If there is an available line, the controller  222  transmits the outgoing call to the PSTN through the available line connected to the line-processing module  230 . 
       FIGS. 2A and 2B  are flowcharts illustrating an emergency calling method in a private branch exchange system according to the invention. 
     Reference will be made to  FIGS. 2A and 2B  in order to sequentially describe an emergency call processing method of the invention corresponding to the operation of the above-described emergency call processing apparatus in a private switching system of the invention. 
     As shown in  FIG. 2A , emergency telephone number information is synchronized among the plurality of subscriber terminals  100 , . . . ,  100   n -1 and  100   n  and the private branch exchange  200 , and are stored in each memory thereof in S 101 . The synchronization of emergency telephone number information will not be described in detail as it has been described previously. 
     With the emergency telephone number information having being synchronized and stored in the subscriber terminals  100 , . . . ,  100   n − 1  and  100   n  and the private branch exchange  200 , the subscriber terminal  100  determines whether digits are inputted from the user in S 102 . 
     If the digits are inputted, the subscriber terminal  100  determines whether it is in a locking mode in S 103 . 
     If it is determined that the subscriber terminal  100  is not set in the locking mode, the subscriber terminal  100  transmits inputted digit values sequentially to the private branch exchange  200  in S 104 . 
     Then, in S 105 , the private branch exchange  200  compares the digit values transmitted from the subscriber terminal  100  sequentially to respective digit values corresponding to one or more emergency telephone numbers stored in the memory  223  thereof. 
     If the inputted digit values do not match those corresponding to one of the emergency telephone numbers, that is, if the telephone number inputted from the subscriber terminal  100  is not one of the telephone numbers registered as emergency telephone numbers, the private branch exchange  200  determines whether there is a currently available line in S 107 . 
     If there is a currently available line, the private branch exchange  200  transmits an outgoing call for the telephone number corresponding to the inputted digits to the PSTN via the available line in S 109 . On the other hand, if there is not a currently available line, the private branch exchange  200  transmits a busy tone signal to the subscriber terminal  100  which has sent the inputted digits in order to inform the user that the outgoing call cannot be processed in S 108 . 
     If the inputted digit values match those corresponding to one of the emergency telephone numbers in S 106 , the private branch exchange  200  determines whether a currently available line exists in S 110 . 
     If there is a line available, the private branch exchange  200  transmits an outgoing call for an emergency telephone number corresponding to the inputted digits to the PSTN via the available line in S 111 . 
     However, if there is not a line available and all lines are busy in S 110 , the private branch exchange  200  intercepts a call on a busy line and transmits an outgoing call corresponding to the emergency telephone number to the PSTN via the intercepted line in S 112 . The process for intercepting a busy line where no lines are available will not be described again since it has been described previously. 
     In the meantime, if the subscriber terminal  100  is currently set in the locking mode in S 103 , the subscriber terminal  100  compares the inputted digits sequentially to those corresponding to the emergency telephone numbers stored in the memory  140  in S 201 . 
     Those digits proved to be a match as a result of comparison are sequentially displayed on the display unit  130  in S 202 . In this regard, if the first digit value does not match, the digit values are determined to be those of a password for disabling the locking mode rather than those of one of the emergency telephone numbers, and the inputted digits are masked and displayed on the display unit  130 . 
     All of the inputted digit values compared in S 201  above are judged as to whether or not they match those corresponding to the emergency telephone numbers stored in the memory  140  in S 203 . That is, it is determined whether the telephone number inputted by the user matches one of the emergency telephone numbers stored in the memory  140 . 
     If the inputted digit values match those corresponding to one of the telephone numbers stored in the memory  140 , the subscriber terminal  100  disables the locking mode, and then transmits the inputted digits to the private branch exchange  200  for the purpose of emergency call origination in S 204 . 
     After the inputted digit values for emergency call origination are transmitted to the private branch exchange  200  as above, process steps of S 110 , S 111  and S 112  in  FIG. 2A  are carried out so as to process an emergency outgoing call. When the subscriber terminal  100  is in the locking mode, if emergency call communication is terminated through emergency telephone number input, the private branch exchange  200  transmits an emergency call termination signal to the subscriber terminal  100  so that the subscriber terminal  100  can be converted to the locking mode again. 
     However, in S 203  of  FIG. 2B , if all of the inputted values do not match those corresponding to the emergency telephone numbers stored in the memory  140  in S 203  of  FIG. 2B , the subscriber terminal  100  determines the inputted digit values to be those for a password for disabling the locking mode rather than for any of the emergency telephone numbers, and masks and displays all of the inputted digits on the display unit  130  in S 205 . In this regard, the masked inputted digit values include those displayed on the display unit  130  due to matching of the initial digit. 
     Then, the inputted digit values are compared to those stored in the memory  140 , which correspond to a password for disabling the locking mode in S 206 , and their equality is determined in S 207 . 
     If the inputted digit values are determined to not match those stored in the memory  140  and corresponding to a password, the subscriber terminal  100  displays a message on the display unit  130 , requesting the user to re-input the password, in S 208 . 
     On the other hand, if the inputted digit values are determined to match those stored in the memory  140  and corresponding to a password in S 207 , the subscriber terminal  100  disables the locking mode in S 209 , and sequentially transmits digits, which are inputted by the user since then, to the private branch exchange  200  in S 210 . 
     After digits for the inputted telephone number (that is, a general telephone number rather than an emergency telephone number) are transmitted to the private branch exchange  200 , the private branch exchange  200  carries out steps S 207 , S 208  and S 209  so as to process an outgoing call for the inputted telephone number. 
     The emergency call processing apparatus and method in a private switching system according to certain embodiments of the invention have been described above. Nations have some emergency telephone numbers, such as 119 and 911, set for emergency. In general, a private branch exchange has more extension subscribers than office lines, and thus the extension subscribers share the office lines. If an emergency outgoing call is attempted when all of the office lines are busy, priority is given to the emergency outgoing call, and one of the busy lines is forcibly intercepted so that the emergency outgoing call can be processed. As an advantageous result, an emergency can be rapidly coped with. 
     In particular, when an extension terminal is set in a locking mode, an emergency call can be processed without inconveniently disabling the locking mode through the input of a password, and thus an emergency can be easily coped with. Then, the emergency outgoing call can be easily processed, even if all of the lines are busy or the subscriber terminal is set in the locking mode. 
     While the present invention has been shown and described in connection with the preferred embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.