PATENT ABSTRACT
A cordless telephone includes a ring detector state machine module to facilitate the answering of a call received at the cordless telephone. A screening state machine module enables three-way connection and aural monitoring of the incoming call on the cordless telephone.

PATENT DESCRIPTION
BRIEF DESCRIPTION 
     The subject of this invention relates to the telephony industry. Specifically, this invention concerns implementation of incoming call screening on a customer premises equipment (CPE) for subscribers of a central voicemail service. 
     BACKGROUND OF THE INVENTION 
     Call screening refers to a called party&#39;s ability to aurally monitor a calling party without the calling party knowing that they are being listened to. Typically, called parties, or users, who desire such a function as call screening must use a telephone answering device (TAD), some other device such as a personal computer (PC), a live attendant, or a combination of these methods to provide the screening activity. The present invention stems from the need for a more cost effective and user efficient method of implementing the call screening function. 
     The use of dedicated call screening equipment such as a PC or a private branch exchange (PBX) requires significant investment in capital equipment and, in many cases, an attendant to qualify, or screen and transfer incoming calls. These methods, while successful, have limitations and/or disadvantages. For example, the hardware and software associated with dedicated equipment can be expensive. Additional costs such as salaries and maintenance fees can be high as well, adding to the overall cost of ownership. While a TAD offers the screening function at a reasonably low cost, it does so at the expense of user efficiency and flexibility. This is so because the user must be in close physical proximity to the TAD in order to listen to the incoming call, thus the TAD method suffers from a lack of user flexibility by limiting use of the call screening function to a specific location. 
     As a result of the breakup of AT&amp;T in 1983 due to Federal antitrust action, the elimination of Bell Telephone as the single telephone service provider created several regional telephone service providers. Responding to increasing consumer competition these regional providers, referred to as the “Baby Bells,” began to offer services other than basic telephone connection. One of these services was a central office (CO) voicemail service. This centrally operated service allowed subscribing users to receive voicemail without the need for any special equipment at their premises. A plain old telephone service (POTS) CPE was all that was required, greatly reducing the cost of ownership for a voicemail service. 
     To use the service a user simply dialed into their account and operated the central system via a set of commands. The user could listen to messages, save, delete, forward or otherwise manipulate messages from their premises. However, use of the centrally served voicemail had a disadvantage. The user could no longer listen to the message, or screen calls as they were being left, an important capability available with a TAD or other call screening method. 
     As mentioned briefly above, there are at present a number of methods for accomplishing the call screening function in use, including CO based call screening service using centrally operated voicemail. For example, in one method, as long as the screening function is enabled by the user, the CO redirects all incoming calls to the user&#39;s central voicemail service. When an incoming call is received the CO signals the user, who then must enter a dual tone multi-frequency (DTMF) code if he wishes to take the call. Thus a significant disadvantage of this method is the necessity for the user to enter a code each time he wishes to selectively screen a call. Obviously, errors in entry of the code or failure to react to the CO signal will cause the user to lose the opportunity to screen an incoming call. 
     In a second method the screening function, if selected by the user, is enabled via a radio link to the user&#39;s cordless CPE. Disadvantageously, however, this method only works for incoming calls supporting caller identification (CID) and requires that the user pre-program a table containing all callers that are to be screened. Thus this method suffers from the dual disadvantages of increased complexity, i.e. the need to program numbers to be screened, and reduced user flexibility, i.e. the inability to screen calls not supported by CID. 
     Yet a third method presently in use involves the use of a TAD to accomplish a switching function in the user&#39;s premises. With this method the TAD is placed between the user&#39;s CPE and the public telephone line. An incoming call may be screened at any time by entry of a code by the user. A second public telephone line must also be attached to the TAD in order for the method to create a three-way call between the calling party, the user, and the CO. This method suffers from numerous disadvantages. First, two separate incoming public telephone lines are required, increasing the expense and complexity of the method. Second, each extension CPE within the user&#39;s premises must be attached to a separate port on the TAD thereby increasing wiring cost, complexity, and the resultant decrease in reliability. Third, the screening function is not automatic. Each time the user wishes to screen an incoming call, a code must be entered which hampers efficient call screening activity. 
     As can be seen, all of the current methods suffer from one or more drawbacks. A further drawback common to all of the present methods is the inability to use the screening function with a cordless phone. A better method would be to provide a way for the user&#39;s CPE to automatically establish a three-way call between the calling party, the user and the user&#39;s central voicemail service without the need or any user intervention or special equipment. A further improvement would be for that method to allow use with cordless phones. 
     SUMMARY OF THE INVENTION 
     A cordless telephone includes a ring detector state machine module to facilitate the answering of a call received at the cordless telephone. A screening state machine module enables three-way connection and aural monitoring of the incoming call on the cordless telephone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a high level block diagram of a system which can make use of the method of the present invention. 
     FIG. 2 is a detailed block diagram of a typical user premises CPE which can make use of the method of the present invention. 
     FIG. 3 illustrates Primary/Alternate screening unit actions on a telephone line using the method of the present invention. 
     FIG. 4 illustrates a top level flow chart of the process implemented by the method of the present invention. 
     FIG. 5 illustrates a ring Detector state machine of a CPE embodiment which can make use of the present invention. 
     FIGS. 6A,  6 B,  6 C,  6 D,  6 E and  6 F: illustrates a Screening state machine of a CPE embodiment which can make use of the present invention. 
     FIG. 7 illustrates a Ring Detector state machine of a CO embodiment which can make use of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     As described briefly above, there are a number of disadvantages with the current methods for call screening, either locally using a device such as a TAD, a PBX, or through a central voicemail service. Primary among these are the need for user intervention to enable screening, the need to be at or near the screening device, and the expense and complexity associated with multiple telephone lines and extension wiring. The present invention overcomes these disadvantages by providing a method for automatically enabling incoming call screening by establishing a three-way connection with the user&#39;s central voicemail service. Once the screening function has been configured, either locally at the user&#39;s CPE or remotely at the CO, the function is always available and allows the user the flexibility to screen or intercept the call with no special action required. Thus the method of the present invention overcomes the lack of flexibility and wiring complexity disadvantages of other methods at a cost similar to the TAD method. 
     The method of the present invention further overcomes the disadvantages enumerated above by providing the incoming call screening function using standard telephony signaling techniques, thus no special hardware is required. Advantageously, the present invention has the ability to recognize different incoming signal types, activating the call screening function only when a normal incoming call is received. Thus when a message waiting signal is received from the CO, for example, the method of the present invention does not activate call screening, but allows the CPE to respond as it normally would to this type of signal. 
     For a more complete understanding of the method of the present invention, it is necessary to first describe the environment in which the invention operates. Referring to FIG. 1, a high-level block diagram  100  of a typical system that can make use of the method of the present invention is shown. A user&#39;s premises  150  contains a Primary Screening Unit (PSU)  155  and an Alternate Screening Unit (ASU)  155 A. A screening unit is the PSU if it is the device programmed by the user to be the first to engage the screening function and will have a hook switch  160  and may have a speaker switch  166  and/or a mute switch  168  if the device is of the type normally equipped with such switches. However, while all CPEs must have a hook switch  160 , it is not necessary for any screening unit to have either a speaker switch  166  or a mute switch  168 . If the screening unit is of the cordless type, the handset may have a handset mute switch and a talk switch that perform essentially the same functions as their counterparts on the PSU device  155  in FIG.  1 . In any case, these switches perform functions well understood by those of skill in the art. 
     Both of these screening devices are connected in parallel to telephone lines  180 , which are part of the public switched telephone network (PSTN)  170 . Note that while the present invention makes use of a combination of two CPE devices, both of which are base station, or corded type telephone equipment, as is known to those skilled in the art, this is not the only possible CPE device configuration that may take advantage of the method of the present invention. For example, one or more of the CPE devices could be a cordless type telephone. Further, as will be discussed in detail below, only the PSU need contain the processing intelligence for the method of the present invention. 
     Also connected to the PSTN  170  via similar telephone lines  180  are the CO  130  and the calling party&#39;s premises  110 . The CO  130  has a central switch  135  that contains, among other functions, the user&#39;s central voicemail service. In one embodiment of the present invention, the central switch  135  further contains the necessary intelligence to implement the method of the present invention. The calling party&#39;s premises  110  contains a telephone equipment  120  used to initiate a call to the user. 
     Referring now to FIG. 2, a detailed block diagram  200  of a CPE device that can typically make use of the method of the present invention is show. In this first embodiment of the present invention a CPE device that is a corded type telephone equipment is used, but as is known to those skilled in the art, the method of the present invention may be implemented in a number of different CPE devices, for example, a TAD, a PC, or other systems comprised of the requisite hardware and software. Note that there are numerous corded telephone devices that could make use of the present invention including a speakerphone or a stationary telephone with a standard handset. The balance of the discussion of the first embodiment of the present invention assumes the corded device is a speakerphone, however, this should not be read as a limitation on the scope of the invention. 
     In FIG. 2 a CPU  210  is connected to a memory  220  via address and data buses in a manner well understood by those of skill in the art. The memory  220  contains, among other data, a ring detector state machine  224  and a screening state machine  228 . These two state machines supply the necessary intelligence in the form of process code to implement the method of the present invention and are discussed in detail below. However, as is known to those of skill in the art, state machines are only one possible method for representing the instructions stored in memory used to accomplish functions in a digital device such as that shown in FIG. 2 thus should not be read as a limitation on the method of the present invention. 
     Also appearing in FIG. 2 are control logic circuits  230  and input/output circuits (I/O)  240 . Each of these functional blocks accomplishes those I/O tasks associated with the operation of a digital device and will be well understood by those skilled in the art. Line interface  205 , display and indicators  260  and keypad  250  also provide functions well known to those skilled in the art. Voice chip  255  is used to play an outgoing message in response to an incoming call that has been answered by the user&#39;s CPE. In one embodiment the chip is a C5212 from General Integrated Systems, San Jose, Calif., USA. 
     As described above, the method of the present invention establishes a three-way call between the calling party, the user, and the user&#39;s central voicemail service. This three-way connection is created after an outgoing voice message (e.g. “Please hold while we complete your call”) has played informing the calling party that the call is being transferred. In reality the call is being connected to the user&#39;s voicemail but, as is known to those skilled in the art, any combination of outgoing message and connection point may be used without departing from the spirit of the invention. 
     FIG. 3 depicts a series of actions  300  showing the reaction of the PSTN to the method of the invention. The upper series of actions  310  to  370  are completed by the Primary Screening Unit (PSU)  155  in FIG. 1, while the lower series of actions  310 A to  370 A are completed by the Alternate Screening Unit (ASU)  155 A of FIG.  1 . Note that there may be more than one ASU without departing from the spirit of the invention. 
     When an incoming call is received, the PSU qualifies the incoming ring at  310 . At the same time the ASU also qualifies the ring at  310 A in case it is the unit that the user takes off hook to intercept the call. Ring qualification is needed to allow the type of incoming call to be determined and is accomplished using a series of software timers in a way well understood by those of skill in the art. For example, the incoming call may be a message waiting ring signal, which is a series of two short rings approximately 0.8 seconds in duration separated by approximately 0.4 seconds, compared to a single approximately 2.0 second ring for a normal incoming call. The screening units must be capable of recognizing special incoming ring signals as something other than a call that requires screening. 
     Having qualified the incoming ring as a call requiring screening, at  320  the PSU seizes the line and at  330  begins playing the outbound voice message telling the caller that the call is being transferred. At the same time the ASU monitors the line looking for a line-in-use condition at  320 A. Upon seeing the line seizure, the ASU qualifies the time required to complete the outgoing message at  330 A. After the outgoing message has completed the PSU flashes the line at  340 . The CO reads the flash and places the calling party on hold. Once on hold the PSU then dials the user&#39;s central voicemail service at  350 . During the same period of time the ASU looks for the flash at  340 A and then qualifies the time needed to complete the connection to the central voicemail service at  350 A. After the connection to the central voicemail service has been made, the PSU again flashes the line at  360 , and, as with the first flash, the ASU looks for the second flash at  360 A. Once the second flash has completed at  370 , the three-way call is established and both the PSU and ASU go to the speaker mute mode at  370 A. At this point the user can hear the calling party without the calling party&#39;s knowledge and has the option of intercepting the call from either the PSU or the ASU by simply taking the device off hook. 
     The method for establishing call screening discussed above is accomplished by operational code stored in the memory of the PSU. The operational code may be described in detail through a discussion of the state machines appearing in FIGS. 5 through 7 in conjunction with FIG. 3 above, however, FIG. 4 provides a top level flow chart  400  of the method of the present invention. As shown, the process is in the Idle state at step  410  when a ring is received at the user&#39;s CPE indicating the presence of an incoming call. In step  420  the ring is detected by the CPE or CO, depending on the embodiment, and at step  430  the ring is qualified to insure that it is truly an incoming call subject to the screening function. An example of a ring that is not an incoming call would be a message waiting ring. 
     Once qualified as an incoming call, an outbound voice message is played at  440  telling the calling party that their call is being transferred. After the outgoing message has played a three-way call is established at step  450 . This is accomplished by flashing the line a first time, dialing the user&#39;s central voicemail service, then flashing the line a second time. When the three-way call has been established call screening commences as at step  460 . Screening may be terminated in a number of ways, for example, the user intercepting the call, but in any case the process returns to the Idle state at  470  where it waits for the next incoming call. 
     The balance of the detailed discussion of the method of the present invention centers on the state machines appearing in FIGS. 5 through 7. For the remainder of the discussion, where a timer is described, it may be assumed to have an accuracy of +/−2.0 milliseconds unless specifically stated otherwise. Signal and event names appear in sans-serif font, for example, START_SCREENING. 
     Beginning with FIG. 5, the CPE based ring detector state machine  500  is shown. Note that the difference between the CPE based embodiment of the present invention and the CO based embodiment lies in the detail of the ring detector state machine. The CPE embodiment will be covered here while the CO embodiment will be described in detail later in the discussion. The ring detector state machine is in the Idle state at  510  when an incoming ring is received. A RING_ON signal is generated at  512  and, if the user has enabled the call screening function, a sys:bCSType=none event  513  occurs and the process returns to Idle. Absent the occurrence of the sys:bCSType=none event  513  a ring counter nRings is set to one at  514 . At  516  a 5.0 second timer is started and the Ringing state  520  is entered. When ringing has ceased, a RING_OFF signal is generated at  522  and the Ringing Quit state is entered at  530  unless an if nRings=user choice event  524  occurs indicating that the number of rings in the ring counter nRings is equal to the user&#39;s preset number of rings. If this occurs a START_SCREENING:PRIMARY signal is generated at  526 , the Ringing state is exited, and the ring counter nRings is reset to zero at  528 . 
     The 5.0 second timer started at  516  times out and generates a TIMEOUT signal at  532 . The purpose for this timer is to provide a ring qualification time window of sufficient length to mask distinctive ring cadences such as the message waiting signal described above. The 5.0 second timer is then restarted at  534  and the Ringing state  520  is again entered. At this point the RING_ON signal is again generated in response to the next incoming ring signal, this time at  545 , the ring counter nRings is incremented at  540  to reflect that the next incoming ring signal has occurred and the Ringing state  520  is reentered. 
     The process continues to alternate between the Ringing and the Ringing Quit states until either the user&#39;s preset number of rings has occurred or until the 5.0 second timer times out without reaching the preset number of rings at  518 . If this occurs, the process returns to the Idle state at  510 . However, at any time a phone is taken off hook a LIU_TRUE signal is generated at  550 . This could occur, for example, if the user wished to intercept the call. If a LIU_TRUE signal is generated and if the ring counter nRings equals or exceeds two rings an if nRings&gt;=2 event  555  occurs indicating that the user may wish to screen the call from an extension phone, thus a START_SCREENING:ALTERNATE signal is generated at  560  and the Ringing state is exited. 
     As with the ring detector state machine just described, the screening state machine  600  starts in an Idle state at  610 , as shown in FIG.  6 A. If the screening state machine receives a START_SCREENING:ALTERNATE signal, such as at  682 , a 4.0 second timer is started at  684  and the Alt 1  state is entered at  690 . As will be discussed in greater detail below, at this point in the process a series of five sequential states begins to check for a LIU_TRUE signal, which could indicate that the user has elected to intercept the call at an extension CPE. 
     If a START_SCREENING:PRIMARY signal is generated, such as at  612 , the line is seized at  613  and a 2.5 second timer is started at  614 . In preparation for screening, speakerphone type CPEs are set to speaker mute mode at  616  and, in the case of cordless or non-speakerphone phones, at  618  the handset mute mode is set. At  620  the intercept flag is cleared, at  622  the voice chip is turned on and the Voice state is entered at  630 . It is at this point that the method of the present invention is playing the outgoing message to the caller for approximately 2.5 seconds indicating that the call is being transferred. While the time for the outgoing message to play in the embodiments of the present invention is 2.5 seconds, it should be clear to those of skill in the art that more or less time may be used without departing from the spirit of the invention. 
     At  652  the 2.5 second timer has expired causing a TIMEOUT signal to be generated and the voice chip turned off at  654 . The line is flashed at  656  and a one second timer started at  658 . The 1.0 second timer is used to allow sufficient time for the CO to react to the flash and deliver a dial tone to the user&#39;s CPE. The process enters the Flash One (F 1 ) Start state at  660  preparatory to checking to see that all user CPEs are on hook. 
     If the user intercepts the call at any time the screening process is halted. For example, if the user lifts the handset, presses the speaker key or the base mute key on a speakerphone type of CPE, a HOOK_SW_UP, SPEAKER, or BASE_MUTE signal respectively is generated at  642 , the 2.5 second timer is cancelled at  644  and the voice chip is turned off at  646 . The Speaker Mode state is entered at  650  and the user handles the call as a normal telephone conversation. In a similar manner, if the user lifts the handset of a cordless phone or a non-speakerphone type phone off of its cradle, or presses the talk or mute keys, a HOOK_SW_UP, TALK, or HANDSET_MUTE signal respectively is generated at  632 . The 2.5 second timer is canceled at  634 , the voice chip turned off at  636  and the Handset Mode state is entered at  640 . As with the speakerphone example previously described, the user now engages in a normal telephone conversation with the calling party. 
     If at any time the user presses the hold key on any type of CPE, a HOLDKEY signal is generated at  672  and the phone is placed on hold at  674 . In this way the user may allow the screening process to continue without hearing the incoming call. This permits the calling party to leave a message in the normal manner for later review by the user. 
     Turning now to FIG. 6B, the F 1  Start and F 1  End states are shown. Note that the F 1  Start and F 1  End states are part of the screening state machine  600 . The F 1  Start state  710  has been entered as a result of the flash generated at  656  in FIG.  6 A. The purpose of this state is to verify that all user CPEs are on hook and that the line is not in use. If this condition exists, a LIU_FALSE signal is generated at  712 , a 2.0 second timer is started at  714  and the F 1  End state is entered at  760 . However, if the user lifts the handset on a speakerphone type of CPE or presses the talk switch on a cordless type CPE a HOOK_SW_UP or TALK signal respectively is generated at  730  followed by a SPEAKER, BASE_MUTE or HANDSET_MUTE signal as appropriate at  732 . The INTERCEPT flag is set at  734 , the process will abort the dialing, select the proper audio path, flash the line and make a normal two-way connection. 
     If the user presses the hold key a HOLD signal is generated at  720  and the phone is placed on hold at  722 . As was described above, this condition may exist if the user wished not to hear the incoming call. In this state, the call screening process continues and the calling party is able to leave a message in the user&#39;s voicemail in the normal manner. If the user has intercepted the call by picking up either an extension or the base CPE, the 1.0 second timer which was started at  654  in FIG. 6A will time out causing a TIMEOUT signal to be generated at  740 . A csTimeout=KILL_CS_NO_FLASH event occurs at  742  followed by the start of a 1.0 second timer at  744  and the process returns to the Idle state  750 . 
     Returning to the F 1  End state at  760 , and recalling that a 2.0 second timer was started at  714 , at  773  the 2.0 second timer expires and at  775  the process begins the read of the user&#39;s voicemail service telephone number. At  780  a READ_DONE signal is generated indicating that the system has completed the read of the number. If the LIU_FALSE condition exists, the handset speaker earpiece is muted at  782  and at  784  the user&#39;s voicemail number is dialed. A 2.0 second timer is started at  786  and the Dialing state is entered at  790 . The 2.0 second timer started at  786  provides sufficient time to dial a seven or eleven digit outbound telephone number. If for some reason the intercept flag was set, as at  792 , the line may be flashed at  794  and the appropriate telephone mode entered, for example, the Handset Mode  796  for a cordless type telephone or the Speaker Mode  798  for a speakerphone type telephone. The line is flashed at  794  to drop the line that would have been used to call the user&#39;s voicemail service, creating a normal two-way telephone call. 
     While in the F 1  End state, if the user presses the hold key a HOLD signal is generated at  762  and the phone is placed on hold at  764 . Again, as described above, this condition may exist if the user wished not to hear the incoming call. In this state, the call screening process continues and the calling party is able to leave a message in the user&#39;s voicemail in the normal manner. 
     If the user lifts the handset on a speakerphone type CPE the HOOK_SW_UP signal is generated at  770 , followed by a SPEAKER or BASE_MUTE signal at  772 . The INTERCEPT flag is set, as at  774 , the process will abort the dialing, select the proper audio path, flash the line and make a normal two-way connection. In a similar fashion, if the user presses the talk button on a cordless type of CPE the TALK signal is generated at  770 , followed by a HANDSET_MUTE signal at  772 . In either case, the INTERCEPT flag is set, as at  774 , the process will abort the dialing, generate a csTimeout=KILL_CS_WITH_FLASH signal at  776 , start a one second timer at  778 , select the proper audio path, flash the line and make a normal two-way connection. As will be discussed in detail below, the csTimeout counter may be forced to arbitrary states in order to direct process flow. At  776  the counter is forced to a state beyond the maximum allowed screening time value in order to force the process to abort the screening operation and enter a normal two-way call mode at  792 . 
     Referring now to FIG. 6C, the Dialing state  810  is shown. Note that the Dialing state  810  is part of the screening state machine  600 . The 2.0 second timer which was started at  786  in FIG. 6B times out at  812 . At  814  the speaker in the handset is unmated and a 1.0 second timer is started at  816  in preparation for establishing a three-way connection, and the Wait Flash state is entered at  850 . The 1.0 second timer is needed to allow enough time for the CO to be ready to accept the upcoming flash soon after the completion of the outbound dialing sequence. If for some reason the outbound telephone number has not completed the dialing process, a still dialing event  840  occurs, a 100 milli-second timer is started at  842  and the process again enters the Dialing state  810 . This loop continues until the outbound number has completed the dialing sequence. Steps  820  through  834  operate in the same fashion and accomplish the same functions as steps  720  through  734  in FIG.  6 B and are not discussed in detail here. 
     Returning now to the Wait Flash state  850 , the 1.0 second timer started at  816  times out resulting in a TIMEOUT signal at  862 . The line is flashed for a second time at  864 , a 1.5 second timer is started at  866 , and the Flash Two (F 2 ) Start state is entered at  870 . The purpose of the 1.5 second timer is to allow the CO enough time to recognize the second flash from the user&#39;s CPE. Steps  852  through  856  operate in the same fashion and accomplish the same functions as steps  820  through  834  discussed above and are not discussed in detail here. 
     The F 2  Start state  910  and the F 2  End state  950  are part of the screening state machine  600  and appear in FIG.  6 D. Note that the process associated with steps  920  through  934  and  960  through  974  operate in the same fashion and accomplish the same functions as steps  820  through  834  discussed above and are not discussed in detail here. Recalling that a 1.5 second timer  866  in FIG. 6C was started just prior to entering the F 2  Start state, if no user CPEs have been taken off hook a LIU_FALSE signal  912  occurs, followed by the start of a 1.5 second timer  914 , and entry into the F 2  End state  950 . As with the F 1  Start and End states [ 710  and  760  of FIG.  6 B], the purpose of these states is to allow enough time for the CO to recognize and react appropriately to the flash sent by the user&#39;s CPE. 
     Exiting the F 2  End state  950 , the 1.5 second timer started at  914  times out at  952 , followed by the start of a 1.0 second timer at  954 . This 1.0 second timer is a general purpose clock that will keep track of the screening time and other call variables. The dial tone is enabled at  956  in order to determine if the calling party hung up during the second flash and the Screening state  980  is entered. At this point in the process the user is able to hear the calling party while a message is being left in the user&#39;s voicemail, and, as described above, may intercept the call at any time. Should the user intercept the call, a normal two-way connection will be established and the conversation will continue as a normal telephone call. 
     Returning briefly to the F 2  Start state  910 , if the LIU_FALSE signal  912  does not occur before the 1.5 second timer started at  866  in FIG. 6C expires, a TIMEOUT  940  occurs indicating that a user CPE has gone off hook. As a result, a csTimeout=KILL_CS_NO_FLASH signal  942  occurs, followed by the start of a 1.0 second timer  944 . Once in the Screening state  980 , the process determines how to react to the csTimeout=KILL_CS_NO_FLASH signal  942 . 
     The Screening state  1010  in FIG. 6E is also part of the screening state machine  600 . In this state the three-way call between the user&#39;s CPE [ 155  in FIG.  1 ], the user&#39;s central voice mail server [ 135  in FIG.  1 ] and the calling party&#39;s CPE [ 120  in FIG.  1 ] has been established and the user is actively screening the incoming call as the message is being left. The user may intercept the call at any time. Steps  1020  through  1034  operate in the same fashion and accomplish the same functions as steps  720  through  734  in FIG.  6 B and are not discussed in detail here. 
     It is possible that the calling party disconnects at some point during the process of outgoing message playback or during outbound dialing to the central voicemail number. Recall that at the point where the second flash has ended establishing a three-way call between the calling party, the user&#39;s central voicemail and the user&#39;s CPE, a HOLD_RELEASE signal such as that shown at  1040  was generated. The dial tone detector is enabled by a DIAL_TONE ON signal at  1042 . If a dial tone is detected the calling party has disconnected so the line is flashed at  1044 , the user&#39;s CPE is hung up at  1046 , and the process returns to the Idle state  1080 . 
     During the time that the process is in the Screening state  1010 , the process executes a loop in which a 1.0 second general purpose timer continuously expires at  1011  and is then restarted at  1021 . The loop and timer are used to monitor the state of the process during screening activity. Each time the 1.0 second timer expires, the csTimeout counter  1012  is incremented. In this way, the process tracks the time spent in the Screening state  1010 . For example, when the csTimeout counter equals one, the dial tone detector is enabled as a dial tone detector at  1014 , but when the csTimeout counter equals four, the same dial tone detector is enabled as a voice detector, as at  1016 . Note that the methods used to start, stop, count and monitor the 1.0 second timer are well known to those of skill in the art and are thus not discussed in detail here. 
     Beginning at  1011 , when the 1.0 second timer has just expired, if there has been no voice activity for 4.0 seconds, or if the csTimeout counter has reached its maximum value, as at  1050 , the line is flashed at  1052 , the CPE is hung up at  1054  and the process returns to the Idle state  1080 . Note that for the present embodiment csTimeout maximum is set at 180 seconds, equivalent to three minutes, however, as will be evident to those of skill in the art, other maximum values are possible without departing from the spirit of the invention. If the process branch conditions of  1050  are not present, the csTimeout counter is incremented at  1012 . 
     If the INTERCEPT flag is set, such as at  1060 , the user has elected to intercept the call. If the user chooses to delete any partial message that has been left, such as at  1065 , the process dials a ** at  1067  to rewind the message (effectively erasing the partial message) otherwise the line is flashed at  1062 , a 3.0 second timer is started at  1064 , the dial tone detector is enabled at  1066  and the Drop Test state is entered at  1070 . In the Drop Test state  1070  the voicemail system is disconnected, or dropped, and the appropriate audio path is selected in the same manner described in detail above. 
     After the first one second timer period csTimeout will equal one, such as at  1013 . The dial tone detector is enabled at  1014  and the next three cycles are used to determine if the calling party has hung up. In this way the process is able to determine if the calling party&#39;s CPE went on-hook during some process event, for example, during the period that a flash was being sent to the CO. Upon the fourth cycle through the screening state process loop, the csTimout counter value will be four, as at  1015 . At this point the dial tone detector is enabled as a voice detector at  1016  and is used to monitor the line for voice activity. This is done by stepping the dial tone detector through various narrow bands of voice frequency looking for low energy presence. If at any point the csTimeout=kill with flash condition occurs, as at  1017 , the line is flashed at  1018  and the csTimeout counter is set to two seconds less than the maximum value, or 178 for this embodiment. The reason for this is that if the process must exit without issuing a flash, setting the csTimeout to 178 will force the exit in two seconds, or when csTimeout counter equals the maximum value of 180. 
     Turning now to FIG. 6F, and recalling FIG. 6A from above, at  682  a START_SCREENING:ALTERNATE signal was received, followed by the start of a 4.0 second timer  684 . This series of actions resulted in entering the Alt 1  state at  690 . FIG. 6F provides the details of the alternate screening states Alt 1  through Alt 5 , which are part of the screening state machine  600 . While the present invention includes five alternate screening states, it should be recognized by those of skill in the art that more or less alternate screening states are possible without departing from the spirit of the invention. 
     After the Screening state machine receives a START_SCREENING:ALTERNATE event, such as at  682  in FIG. 6A, the 4.0 second timer has been stated at  1112  and the Alt 1  state  1114  is entered. Note that the 4.0 second timer  1112  and the Alt 1  state  1114  are the same as those shown in FIG. 6A [ 684  and  690  respectively], but are labeled differently here for clarity in the discussion. The 4.0 second timer allows sufficient time to determine if the PSU has released the line. If a LIU_FALSE event  1116  occurs prior to the expiration of the 4.0 second timer, a 2.0 second timer  1118  is started and the Alt 2  state  1130  is entered. If the 4.0 second timer did expire prior to receiving the LIU_FALSE event  1116 , a TIMEOUT  1120  occurs and the Idle state  1110  is entered. 
     Continuing now with the Alt 2  state  1130 , if a LIU_TRUE event  1132  occurs, the 2.0 second timer is restarted at  1134  and the Alt 3  state  1140  is entered. This series of events indicate that the PSU has commenced the screening process and the outbound dialing to the user&#39;s voicemail is occurring. If the two second timer expired prior to the LIU_TRUE event, a TIMEOUT occurs at  1136  and the process returns to the Idle state at  1110 . In the Alt 3  state  1140  the process monitors extension phones to see if an ASU has gone off hook. If it has, the user has intercepted the call, a LIU_FALSE event occurs at  1146  and the process returns to the Idle state at  1110 . However, if the 2.0 second timer expires, such as at  1142 , the 2.0 second timer is again restarted at  1144  and the Alt 4  state  1150  is entered. In this state the process is looking for a flash from the PSU as it sets up a three-way call between the user&#39;s CPE, the user&#39;s central voicemail server, and the calling party. 
     While in the Alt 4  state  1150 , if the 2.0 second timer expires a TIMEOUT event occurs at  1156 , indicating that this is not a screening call, thus the process returns to the Idle state  1110 . Upon the occurrence of a LIU_FALSE event  1152 , the 2.0 second timer is again restarted at  1154  and the process enters the Alt 5  state  1160 . In the Alt 5  state  1160  the process monitors the line to determine if the time between the LIU_FALSE event  1152  and the LIU_TRUE event  1161  is appropriate for a flash from the PSU. If not, the 2.0 second timer expires at TIMEOUT  1170  and the process returns to the Idle state  1110 . If the LIU_TRUE event  1161  occurs before the 2.0 second timer expires, the primary unit has established the three-way call, and the process configures the CPEs, whether base or cordless, for screening. At  1162  the line is seized, a 1.0 second timer is started at  1164 , the base unit is set to the speaker mute mode at  1166 , the cordless CPE is placed in the handset mute mode at  1168 , and the csTimeout counter, described above in the Screening state, is set to zero at  1169 . The Screening state is then entered at  1180 . Note that the Screening state  1180  is the same as the Screening state  1010  in FIG. 6E, but is labeled differently here for clarity of the discussion. 
     As with other states previously discussed, at any time the user may intercept the call. If the user decides to intercept the call, a HOOK_SW_UP or TALK signal will occur such as at  1192 , followed by a SPEAKER, BASE_MUTE, or HANDSET_MUTE signal at  1194 . Depending upon the type of CPE device intercepting the call, the appropriate audio path is selected and the process enters either the Speaker Mode  1196  or the Handset Mode  1198 . Note that the methods used to react to the signals and select the proper audio path are identical to the same methods discussed in detail above, thus they are not discussed here. 
     The preceding discussion articulates the method of the present invention as implemented in a user CPE. However, the method may also be implemented at the CO. There are at present CO based call screening methods, but these have the several disadvantages that were enumerated above. The method of the present invention allows the CO based screening to be as flexible to the user as the CPE based method. For the CO based method, the Screening state machine is identical to and performs identical functions as in the CPE based method and is not discussed in detail here. The difference is, of course, that the Screening state machine is now located at the CO. 
     The main difference between CPE based and CO based screening methods is the Ring Detector state machine. The reason that the Ring Detector state machine must be different is that in the CO implementation of the method of the present invention, a ping ring, or ring splash, is used to indicate the presence of a calling party to the user. A ping ring is a short burst of approximately 500 milliseconds in duration. Since the incoming ring is different, a modified Ring Detector state machine is required to qualify the ring. In this method the user&#39;s phone first outputs a normal incoming call ring pattern: generally 2.0 seconds of ringing followed by 4.0 seconds of silence. Once the CO has established a connection between the calling party and the user&#39;s central voicemail, a ping ring is generated at the CO letting the user know that a message is being left. At this time the user may wish to screen the call, intercept the call, or allow the calling party to simply leave a message. 
     FIG. 7 shows the modified Ring Detector state machine  1200 . From the Idle state  1210  a RING_ON event occurs at  1215 , causing the Ringing state  1220  to be entered. If the sys:bCSType=none condition exists, the call screening function is being handled by the CPE, thus the process returns to the Idle state  1210 . If the sys:bCSType=none condition does not exist, after the initial ring a RING_OFF event  1222  occurs, followed by the start of a 4.1 second timer at  1224 . This timer is slightly longer than the 4.0 second off time of a normal ring signal. The Ringing Quit state  1230  is entered and the process waits for one of two events: either a TIMEOUT event  1234  from the 4.1 second timer or a new RING_ON event  1232 . If a new ring is received at RING ON  1232 , the call from the CO has not yet been connected to the user&#39;s central voicemail, thus the Ringing state  1220  is re-entered. However, if the TIMEOUT  1234  has occurred, the CO has transferred the call, a 20 second timer  1236  is started and the Look for Ping state  1240  is entered. The 20 second timer is set to be long enough to cover the delay before the CO outputs the ping ring. As is known to those of skill in the art, this delay could be more or less than 20 seconds without departing from the method of the invention. 
     In the Look for Ping state  1240  the process is waiting to determine if the calling party has disconnected. If the TIMEOUT at  1246  occurs, the CO has not sent a ping ring indicating that the calling party has hung up and the process returns to the Idle state  1210 . If a RING_ON event  1242  is detected, a 300 millisecond timer  1244  is started and the process enters the Ping Start state  1250 . Since the ping ring duration is approximately 500 milliseconds, if a RING_OFF event  1256  occurs, the CO has aborted the call and the process returns to the Idle state  1210 . If the 300 millisecond timer expires, a TIMEOUT event  1252  occurs indicating that the incoming ring could be a ping, so a 400 millisecond timer is started at  1254 , and the process enters the Ping End state  1260 . 
     Recalling that the ping ring duration is approximately 500 milliseconds, if a RING_OFF event  1262  is received prior to the expiration of the 20 millisecond timer, a ping ring may have been received, and in response, a 4.1 second timer is started at  1264  and then the process enters the Wait No Ring state  1270 . If the TIMEOUT event  1266  occurs, the incoming ring is greater than the length of a ping ring and the process returns to the Idle state  1210 . For example, this might occur if a new call arrived and the CO has generated a standard incoming ring whose duration would exceed the total of the 700 milliseconds from the RING_ON event  1242  to the Ping End state  1260 . 
     The purpose of the Wait No Ring state  1270  is to qualify the incoming ring as a ping ring. If a new RING_ON event  1274  occurs prior to the expiration of the 4.1 second timer, the ring is not a ping ring and may be part of a new normal ring cadence. This is so since a ping ring is followed by a period of silence greater than the 4.0 second off time of a normal ring cadence. If a new RING_ON event  1274  is received, the process returns to the Idle state  1210 . If the 4.1 second timer expires, a TIMEOUT event  1272  occurs and the process enters the Screening state machine via the START_CALL_SCREENING:AUDIO event  1280 . The balance of the CO call screening method is identical to the CPE method discussed in detail above. 
     One advantage of the present invention is the ability of the user to perform the call screening function automatically, eliminating the need for entering a code or employing specialized TAD equipment. This flexibility is gained through the use of a series of state machines contained in memory in the form of operational program code of either a CPE device at the user&#39;s premises or in an equipment located at the CO. 
     A second advantage of the present invention is the ability to screen calls from any CPE at the user&#39;s premises eliminating the need to be physically close to the CPE device doing the screening. The method of the present invention employs the necessary state machines in operational code to allow the user to accomplish the screening activity from any CPE including base speakerphone type CPEs or cordless type CPEs. 
     A third advantage of the present invention is the ability of the user to choose between standard screening wherein the user can listen to the calling party without the calling party&#39;s knowledge, allowing the calling party to leave a message in voicemail, or to intercept the call at any time from any CPE in the user&#39;s premises without the need to enter a special code. This advantage of the present invention is a significant improvement over earlier methods that did not allow such flexibility for the user. 
     A fourth advantage of the present invention is that no special equipment is required. By eliminating the need for PBX equipment or live attendants, the method of the present invention is very economical. The necessary state machines may be placed in memory that already exists in the majority of user CPE equipments at this time. 
     A further economic advantage is gained by the fact that the necessary operational code containing the state machines need be located in only one user CPE, thus a fifth advantage of the present invention is simplicity of adding the call screening function to extension locations within the user premises. No costly wiring is required as with prior art methods. 
     A sixth advantage of the present invention is that it may be implemented in any system having the appropriate hardware and software. Thus the method of the present invention may be used with a PC configured for telephony operation, or a PBX installed in an office as well as the more general application to CPEs such as telephones and TADs.