hereinbelow , detailed embodiments of the present invention will be described in detail with reference to accompanying drawings . a telephone switch system for performing a conferencing function adopted in the present invention is shown in fig1 which includes a plurality of mobile stations ( ms ) represented by subscribers a , b and c ( specifically , a conferencing function subscriber is represented by a . ), a base transmission station ( bts ) for transmitting and receiving data to / from the plurality of mobile stations ( ms ) in a wireless manner , a base station controller ( bsc ) for communicating with the respective bts &# 39 ; in a wire manner and selecting the corresponding bts , a mobile switch center ( msc ) for directing a selection control of the bsc in a wire manner , providing the data selected by the bsc to a public switch telephone network ( pstn ) or transmitting responsive data to the bsc , a home location registration ( hlr ) for registering a conferencing function subscriber among subscribers selected from the bsc in a no . 7 signaling method through a wire , and the pstn for receiving the wired data from the msc to select subscribers , e . g ., d , e and f , and establishing a communication path of the corresponding subscriber to transmit the same to the msc . in this case , the msc includes a mobile access switch server ( ass - m ) for transmitting and receiving a call to / from the bsc . the ass - m has an extended t1 trunk interface board assembly ( etia ) for interfacing the transmission and reception of the call with the bsc , a mobile multiway controller ( mmul ), an access switch processor for transmitting and receiving the call with the etia , a time switch unit ( tsu ) connected to the etia and a sub - highway ( shw ) to then be connected to a next inter network subsystem ( ins ) and a highway and controlled by a time switch processor ( tsp ), and the time switch processor ( tsp ) connected to the asp to control the tsu , for transmitting and receiving the call with the asp . the msc also includes a call mixer unit ( cmxu ), a global service processor ( gsp ), and an access switch subsystem for subscriber ( ass - s ) having an access switch processor ( asp - g ) for mixing voice between subscribers . further , the msc includes an access switch subsystem for trunk ( ass - t ) for transmitting and receiving a call with a pstn subscriber . the msc guides the voices of both parties from the ass - m and ass - t to be mixed in the ass - s , and includes an inter network subsystem for transmitting the mixed voice to the ass - m and ass - t . the ass - s includes the call mixer ( cmxu ), which is comprised of five conference mixer circuit board assemblies ( cmcas ). fig2 shows a cmca and input / output portions corresponding thereto . the cmca includes a pcm access portion 10 connected to the tsu and shw constituting the ass - m in the msc for interfacing voice data of subscribers , a td - bus access circuit 12 connected to the global service processor 20 , an i / o ram 16 for inputting and outputting data between the td - bus access circuit 12 and a central controlling circuit 14 , a pcm conference mixer device 18 for receiving voice data between subscribers through the pcm access portion 10 and voice - mixing the same to allow a conference , a testing circuit 19 divided from a common node between the central controlling circuit 14 and pcm access portion 10 for testing a fault of a loop path between the subscribers , and an alarm 19 - 1 for detecting faults from the td - bus access circuit 12 , central controlling circuit 14 and pcm access portion and generating an alarm to an impediment processing unit 30 . in this case , the central controller 14 controls the gsp 20 to receive data transmitted to the i / o ram 16 through the td - bus access circuit 12 for performance of conferencing service . the tsu receives pcm data ( dx ) transmitted from the pcm access portion 10 and outputs pcm data ( dr ) to apply the same to the pcm access portion 10 . also , the tsu outputs a clock signal clk and a frame sync signal fs to supply the same to the pcm access portion 10 of the cmxu ( cmca )). in other words , according to the application of a subscriber ( e . g ., a ), the subscriber is registered in the home location registration ( hlr ). the registered subscriber is requested to register in a visitor location registration ( not shown ) in advance . also , the voice is mixed in the cmxu included in the ass - s of the msc so that the ms calls the subscriber to allow a call between both parties or multiple parties . thus , the subscribers engaged in the conference can hear their own voices while attending the conferencing communication . fig3 is a flow diagram showing the conferencing service method according to the present invention , in which the algorithm for the conferencing service is performed in a mobile multiway controller ( mmul ) of the asp in the ass - m installed in the msc . the conferencing service method according to the present invention is performed by the steps of performing a basic communication by a conferencing communication or a general telephone communication between subscribers ( step s1 ), conferencing with a conferencing function subscriber by selecting the conferencing function during the basic communication ( step s2 ), making an existing conference subscriber wait and communicating with another subscriber if there is a call request from another subscriber during the conferencing communication ( step s3 ), adding another subscriber during the conferencing communication and allowing the conferencing communication to be continued ( step s4 ), conferencing again after the steps s3 and s4 ( step s5 ), checking whether the conferencing function subscriber generates an subscriber deleting id and performing the conferencing procedure of the step s2 if not ( step s6 ), selectively performing a subscriber deleting procedure if the subscriber deleting id is generated in the step s6 and then going back to the conferencing step s2 ( step s7 ), and deleting / adding subscribers ( step s8 ). during the conferencing communication according to the present invention , in the call waiting step s3 , as shown in fig4 while the conferencing communication is performed over telephone , if a busy conference request is received in the mmul from a new subscriber ( e . g ., a subscriber d ) searching after a conferencing function subscriber ( e . g ., the mobile station ( ms ) a shown in fig1 ) ( step s3 - 1 ), the mmul determines whether a call waiting function is offered to the new subscriber proposing the busy conference request ( step s3 - 2 ). it is assumed that the call waiting function of the subscribers is registered in the visitor location registration ( vlr ) controlled by the asp of the ass - m installed in the msc . if the new subscriber is equipped with the call waiting function , the mmul outputs a call waiting signal to the subscriber subject to check for the busy conference request , for example , the conferencing function subscriber ( here , it is assumed that the call waiting function is registered .) to notify that there is the busy conference request ( step s3 - 3 ). accordingly , if the busy conference requested subscriber ( e . g ., the conferencing function subscriber ) presses a transmission key ( e . g ., a &# 34 ; send &# 34 ; key ) ( step s3 - 4 ), the mmul ( hereinbelow , to be broadly referred to as an exchange system ) outputs a call waiting signal to all subscribers attending the conferencing communication to hold the communication ( step s3 - 5 ). then , a new subscriber ( e . g ., a subscriber d ) and the busy conference requested subscriber ( e . g ., the conferencing function subscriber a ) are connected to perform a telephone communication between both parties ( step s3 - 6 ). if the busy conference requested subscriber presses the transmission key again after completing the communication , the exchange system determines whether a transmission signal is input to the mmul of the asp . in such a state , if the transmission key is input again , the exchange system cancels the call waiting signal for all subscribers let to be waited in the course of conferencing communication , and outputs the call waiting signal to a new subscriber ( step s3 - 8 ) to switch to the conferencing communication to resume the conferencing communication with existing subscribers engaged in the conferencing communication ( step s5 ). by such an operation , whenever a transmission key is input , a call waiting service can be repeatedly rendered during conferencing communication . therefore , according to the present invention , if there is a newly received call in the course of conferencing communication over the telephone , the subscriber equipped with a call waiting function ( conferencing function subscriber ) selects a subscriber engaged in the conferencing communication with a calling party of the new call by his / her key manipulation so that telephone communication and conferencing communication are performed . thus , the conferencing function subscriber can be noticed of an urgent contact even during his / her attending in the conference . as shown in fig5 in the step of adding subscribers during conferencing communication ( step s4 ), if there is a call request from another subscriber to the exchange system ( step s4 - 1 ) while performing telephone communication ( step s2 ), the exchange system checks whether the conferencing function subscriber registers a subscriber adding function for setting another subscriber ( step s4 - 2 ). at this time , if the conferencing function subscriber does not register the subscriber adding function for setting another subscriber , the exchange system disregards the input of a call set - up for another subscriber to allow the conferencing communication to be continued ( step s4 - 2 ). meanwhile , if the conferencing function subscriber registers the subscriber adding function for setting another subscriber , the exchange system transmits a call waiting signal to the existing subscriber being engaged in telephone communication ( step s4 - 31 ). then , another subscriber and the subscriber equipped with the subscriber adding function , i . e ., the conferencing function subscriber , are mutually connected ( step s4 - 32 ) to allow both parties to perform telephone communication ( step s4 - 33 ). in such a state , the exchange system determines whether a ` send ` key which is provided in the conferencing function subscriber &# 39 ; s telephone is input , and performs the step s4 - 33 if the send key is not input ( step s4 - 34 ). if the send key is input , the exchange system stops transmitting the call waiting signal to the subscriber whose call is waited ( step s4 - 4 ), and then all parties , i . e ., the conferencing function subscriber , another subscriber and waiting subscriber , are connected so that all parties perform the conferencing communication ( step s5 ). therefore , it is possible to add subscribers one by one in the course of performing the telephone communication , such that the subscriber equipped with the subscriber adding function manipulates a key . in other words , a plurality of subscribers can attend the conferencing communication . further , a subscriber who need not attend the conference may be excluded from the conference according to the option of the conferencing function subscriber . according to the conferencing communication of the present invention , in the subscriber deleting step ( step s7 ), as shown in fig3 if a key signal is input in the course of performing the conferencing communication ( step s2 or s5 ), a provisional step of determining whether the input key is a subscriber deleting request id ( e . g ., to be set as `*`) is performed . in this case , if the subscriber deleting request id is input , the subscriber deleting step s7 is performed . in other words , the exchange system determines whether the subscriber who inputs the subscriber deleting request id is equipped with the conferencing function subscriber deleting function . if yes , the exchange system deletes a specific subscriber corresponding to the subscriber code requested by the conferencing function subscriber and the deleting request key from the conference , and allows the other subscribers to continue the conferencing communication . in other words , in the subscriber deleting step s7 of the present invention , if the subscriber equipped with the subscriber deleting function ( the conference function subscriber ) sequentially presses a `*` key ( the subscriber deleting request id key ), a specific subscriber code key and a deletion key ( e . g ., a ` send ` key ), the exchange system performs a deleting function by deleting the specific subscriber corresponding to the specific subscriber code from the conferencing communication to then return to the conferencing step s2 . here , the specific subscriber code is given in the order of attending in the conferencing communication , where ` 1 ` is given to the subscriber who first requested for the conferencing communication ( the conference function subscriber ), and ` 2 ` is given to the subscriber who is connected with the first requesting subscriber by making a call to the subscriber . specific subscriber codes are set such that serial numbers are given in the order of the subscribers engaged in the conference . the subscriber codes can be utilized in deleting subscribers from the conferencing communication . therefore , while the conferencing communication is performed by using the telephone , specific subscribers engaged in the conferencing communication are deleted one by one such that the conference function subscriber manipulates keys , thereby deleting a specific subscriber who completes a predetermined task during the conferencing communication so that the specific subscriber may not attend the conferencing communication any longer . the subscriber deleting / adding step s8 during the conferencing communication of the present invention , as shown in fig3 is performed in the exchange system ( msc ). as shown in fig6 in detail , while the conferencing communication is performed over the telephone ( step s2 or s5 ), if a key signal from the conference function subscriber is input to the msc ( step s6 - 1 ), it is determined whether the input key is a change ( deletion ) key ( e . g ., `*` key ), and the operation depending on the input key signal is performed if not ( step s6 - 2 ). however , if it is determined that the input key is the change key , the msc checks whether the corresponding change function ( deleting function ) is offered to the conference function subscriber , thereby completing to check whether the subscriber deleting id key ( the deletion key , e . g ., the `*` key ( step s6 ). after the steps s6 - 3 , the exchange system waits until the subscriber code to be deleted is input from the conference function subscriber ( step s8 - 1 ), and deletes the corresponding subscriber if the corresponding code ( to - be - deleted subscriber code ) is input ( step s8 - 2 ). next , it is determined whether a call number to be added if necessary , is input , and continues the conferencing communication if the to - be - added call number is not input ( step s8 - 3 ). if the to - be - added call number is input , the exchange system controls a call waiting signal to be generated to all conferencing function subscribers simultaneously ( step s8 - 4 ), and allows communication by connecting the subscriber of the additionally input call number ( step s8 - 5 ). the msc waits until an arbitrary transmission key ( e . g ., a ` send ` key ) set for necessity of a conference function is pressed , i . e ., until the ` send ` key signal is received , the msc waits ( step s8 - 6 ). if the transmission key is pressed , all subscribers are connected so that the waiting subscriber and added subscriber perform the conferencing communication together ( step s8 - 7 ), thereby completing the subscriber deleting / adding step s8 . here , the conferencing function subscriber code in the step s8 - 1 is given in the order of subscribers attending the conferencing communication , where ` 1 ` is given to the subscriber who first requested for the conferencing communication ( the conference function subscriber ), and ` 2 ` is given to the subscriber who is connected with the first requesting subscriber by making a call to the subscriber . specific subscriber codes are set such that serial numbers are given in the order of the subscribers engaged in the conference . at this time , if the to - be - added call number is not input , the conferencing communication is continuously performed among the undeleted existing conferencing function subscribers ( step s8 - 3 ). if the to - be - added call number is input , the conferencing communication is continuously performed among the undeleted existing conferencing function subscribers , a call waiting signal is transmitted to the other conferencing function subscribers to hold all subscribers , excluding the conferencing function subscriber equipped with both the conferencing function and subscriber deleting / adding step ( sep s8 - 4 ). then , the subscriber of the additionally input call number and the conference function subscriber are connected so that both parties perform the telephone communication between both parties ( step s8 - 5 ) in such a state , if the conference function subscriber presses the transmission signal ( e . g ., ` send ` key ) ( step s8 - 6 ), the waiting existing conferencing function subscribers are all connected ( step s8 - 7 ) so that both parties perform conferencing communication . therefore , it is possible to delete a specific conferencing function subscriber in the course of performing the conferencing communication and to add another subscriber to perform the conferencing communication . an unnecessary subscriber can be deleted during the conferencing communication and a necessary subscriber can be added to perform the conferencing communication .