Abstract:
A cellular interface unit is disclosed and is used in association with a trunk line of a private branch exchange or a key system unit. The cellular interface unit includes a cellular-type transceiver, a power supply and control circuitry. The cellular-type transceiver is used for mobile-to-mobile telephone calls with remote cellular telephone units and for mobile-to-landline telephone calls with remote landline telephone units. The control circuitry is used to convert signals between the cellular transceiver and the private branch exchange or key system unit so that the transceiver and its interconnected PBX or KSU function appropriately and so that use of the trunk line establishes a cellular telephone call, while resembling a landline telephone call through the PBX or KSU. Included within the control circuitry is call disconnection supervision circuitry that is responsive to the receipt of an end of call signal from the mobile telephone switching office. In response to the receipt of such signal, the call disconnection supervision circuitry causes the private branch exchange or key system unit to deactivate its associated trunk line and tear down the telephone call by monitoring a physical quantity attributable to the transceiver associated with the interface unit and responding to a change in that physical quantity. The power supply provides a source of electrical energy for the cellular transceiver and the control circuitry.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is generally directed to a telecommunication network and, more particularly, to a fixed cellular communications system that incorporates the use of a private branch exchange (PBX) or key system. 
     The use of telecommunication networks for business and personal purposes is typically regarded as a necessity of modern living. Use of these networks has increased exponentially over the past few decades. Today, telecommunication networks include, among other things, two major types of conventional telephone systems. 
     The first of these two conventional telephone systems shall be referred to herein as a landline telephone system. Landline telephone systems are the conventional wire-based systems that are found in most homes or businesses. Because these systems are wirebased, to develop such systems, the telephone service provider must acquire rights of way and establish a network infrastructure, which are costly endeavors. These telephone systems are typically regarded as being fixed in that the mobility of a telephone set connected to the system is limited to a relatively short range. 
     The second of these two conventional telephone systems shall be referred to herein as a cellular telephone system. Cellular telephone systems are the conventional wireless systems that have become increasingly popular among telephone service subscribers. The primary advantage of using these systems is that they permit mobility of the telephone handset, which allows the user to be reached even when he is located away from the home or office. The telephone handsets for cellular telephone systems are typically either connected to a cellular transceiver unit mounted in the trunk of an automobile or are portable, hand-held units that include a cellular transceiver in a common housing and can travel with a subscriber in his or her pocket or bag. 
     While the popularity of cellular telephone systems has increased, the expense associated with the use of such systems has steadily decreased, particularly when taking into account the standard rate of inflation. The use of cellular systems, however, is still generally more expensive than the use of conventional landline telephone systems. Cellular service providers, which often are not associated and/or affiliated with landline service providers, could establish and market billing plans that provide incentives for telephone subscribers to place mobile-to-mobile (cellular-to-cellular) telephone calls as opposed to mobile-to-landline and/or landline-to-mobile telephone calls. In many cases, the cost for a cellular-to-cellular call is priced at only about twenty-five percent (25%) of the cost for a similar cellular-to-landline call. The cost of mobile-to-mobile calls is less expensive because it is unnecessary to route such calls through the central office of a landline telephone service provider. Accordingly, the landline service provider does not charge the cellular service provider a service fee for utilizing its central office. To the contrary, mobile-to-landline and/or landline-to-mobile telephone calls are routed through the central office of a landline service provider, which charges a service fee for utilization of its central office. That fee is ultimately passed through and billed to the cellular telephone service subscriber. 
     In today&#39;s marketplace, many firms conduct business by having some employees at a main office and having other employees in motor vehicles out in the field who travel from work site to work site. For present purposes, those employees out in the field will be referred to as “mobile employees” and those employees in the main office will be referred to as “fixed employees”. 
     One example of such a firm is a cable television company. Typically, cable television companies employ servicemen or technicians who drive to a job site, perform a service (such as installation or repair of cable television service at that site) and then drive to the next job site. These servicemen are mobile employees. 
     In other examples, some firms have employees who continually roam. For instance, with respect to a taxi cab service company, drivers (mobile employees) travel from location to location in order to transport passengers, while dispatchers (fixed employees) remain at the firm&#39;s main office and communicate with the drivers. The drivers do not actually stop at a work site to perform a job. Rather, they pick up passengers and transport them to a requested site. It will be appreciated that these examples are nowhere near exhaustive of the firms that have both mobile and fixed employees. It will further be appreciated that in the vast majority of these firms the mobile employees contact fixed employees working at the main office several times during each working day, and vice versa. 
     In these firms, it is impractical for the mobile employees to stop what they are doing each and every time they desire to place a telephone call to a fixed employee at the firm&#39;s office. In particular, for each time a mobile employee is required to contact a fixed employee at the firm&#39;s office, it would take too long and be too disruptive for the mobile employee to drive his/her motor vehicle off the road, park the vehicle, exit from its passenger compartment, approach a coin-actuated telephone, insert the appropriate currency, and dial the office telephone number. Further, it is beneficial if the mobile employees can be reached easily at any time. Accordingly, use of a wireless form of communication by mobile employees provides significant benefits, some of which are noted above. A cellular telephone with transceiver is typically the most practical and desirable form of communication. 
     Given the billing plans offered by many cellular telephone providers, where cellular-to-cellular calls can be completed for a cost substantially less than the cost of similar cellular-to-landline calls, if a firm&#39;s mobile employees use cellular transceivers to place outgoing and receive incoming telephone calls, it makes sense for the fixed employees to communicate with the mobile employees by using cellular transceivers as well. In that way, the firm&#39;s overhead and operating costs can be reduced. 
     In prior art telephone systems, it is known to use a fixed cellular communications system to provide for mobile-to-mobile calls. An example of a fixed cellular communications system can be found in U.S. Pat. No. 4,922,517, issued to West, Jr. et al., the disclosure of which is hereby incorporated herein by reference. Fixed cellular communications systems typically incorporate an interface unit that encodes signals received from a landline telephone unit and decodes signals received from a remote cellular telephone unit. These interface units typically include, among other possible things, a cellular transceiver, logic circuitry and a power supply. The interface unit, when combined with the landline telephone unit, permits the landline telephone unit to function as if it were a cellular telephone unit. Under such circumstances, outgoing calls transmitted to a remote cellular telephone unit and incoming calls received from a remote cellular telephone unit are made on a mobile-to-mobile basis. 
     In light of these prior art fixed cellular communication systems, business firms could incorporate a landline telephone unit and an interface unit for every fixed employee so that calls made to and received from the mobile employees would be made on a mobile  10  to-mobile basis and the firm could take advantage of any applicable cost effective billing plans. Use of an external telephone line for every fixed employee, however, is impractical, especially for those business firms that have a relatively large number of fixed employees. 
     It will be appreciated that business firms often employ a private branch exchange (PBX) and/or a key system unit (KSU) for their telephone service. These systems allow for the switching of calls between the fixed employees on local lines, while simultaneously allowing the fixed employees to share external lines (known as trunk lines). The primary purpose of these systems is to save the additional cost that would otherwise be associated with requiring an outside telephone line for each fixed employee. For purposes herein, the words “private branch exchange” and/or the initials “PBX” shall be interpreted to mean both a private branch exchange and a key system unit. Accordingly, those words and/or initials, when used in the appended claims, shall cover both types of systems literally. 
     Those business firms that use a PBX or KSU for their telephone service could try to employ the interface units of the prior art in those systems to establish a fixed cellular communications system. What those firms would discover, however, is that using the prior art interface units with PBX and/or KSU systems creates several unforeseen problems. Among other things, no call teardown occurs when calls are terminated by the remote cellular telephone unit. As such, the prior art interface units have not, until now, been designed to function properly when used in association with a PBX and/or KSU. 
     As is well known in the art, in cellular telephone systems, upon completion of a mobile-to-mobile call between two cellular telephones whose users subscribe to cellular service offered by the sane cellular service provider, one unit terminates the call by transmitting an appropriate end of call (EOC) signal to the mobile telephone switching office (MTSO) utilized by the cellular telephone service provider. The MTSO receives the EOC signal from that unit and either relays that end of call (EOC) signal to the other cellular unit or generates another end of call (EOC) signal for transmission to the other cellular unit. With respect to fixed cellular communications systems, the cellular transceiver included as part of the interface unit either receives an EOC signal (when the call is terminated at the remote unit) or, alternatively, transmits an EOC signal (when the call is terminated at the landline telephone unit associated with the interface unit). 
     When the call is terminated at the remote unit, which is often the case, the prior art interface units typically do not cooperate with the functionality of PBX and/or KSU systems. In particular, when the cellular transceiver included as part of the interface unit receives an EOC signal from the MTSO, it responds by powering down. Nothing else notable occurs. As a result, the trunk line used for the call within the PBX and/or KSU system remains activated. From the standpoint of the PBX and/or KSU system, the call is never terminated, and the trunk line is never released, thereby rendering it useless. 
     In light of the foregoing, it is desirable to design a fixed cellular communications system for use with a trunk line of a private branch exchange and/or key system unit. 
     It is also desirable to design a fixed cellular communications system that effectuates call teardown when used in association with a trunk line of a private branch exchange and/or key system unit. 
     It is further desirable to design an interface unit for a fixed cellular communications system, wherein the interface unit causes its associated trunk line to be released upon termination of a telephone call at a remote telephone unit. 
     It is yet further desirable to design a fixed cellular communications system that includes call disconnection supervision control circuitry for use in association with a trunk line of a private branch exchange and/or key system unit. 
     It is still further desirable to design such a system that includes call disconnection supervision control circuitry that monitors a physical quantity attributable to its associated cellular transceiver and responds to a transition in the value of that physical quantity by causing its electrically coupled trunk line to be released. 
     These and other objects of the preferred form of the invention will become apparent from the following description. It will be understood, however, that an apparatus could appropriate the invention claimed herein without accomplishing each and every one of these objects, including those gleaned from the following description. The appended claims, not the objects, define the subject matter of this invention. Any and all objects are derived from the preferred form of the invention, not the invention in general. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a cellular interface unit for use in association with a trunk line of a private branch exchange or a key system unit. The cellular interface unit includes a cellular-type transceiver, a power supply and control circuitry. The cellular-type transceiver is used for mobile-to-mobile telephone calls with remote cellular telephone units and for mobile-to-landline telephone calls with remote landline telephone units. The control circuitry is used to convert signals between the cellular transceiver and the private branch exchange or key system unit so that the transceiver and its interconnected PBX or KSU function appropriately and so that use of the trunk line establishes a cellular telephone call, while resembling a landline telephone call through the PBX or KSU. Included within the control circuitry is call disconnection supervision circuitry that is responsive to the receipt of an end of call signal from the mobile telephone switching office. In response to the receipt of such signal, the call disconnection supervision circuitry causes the private branch exchange or key system unit to deactivate its associated trunk line and tear down the telephone call by monitoring a physical quantity attributable to the transceiver associated with the interface unit and responding to a change in that physical quantity. The power supply provides a source of electrical energy for the cellular transceiver and the control circuitry. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following detailed description, reference will frequently be made to the following drawings, in which like reference numerals refer to like components, and in which: 
     FIG. 1 is a diagram illustrating a telecommunication network and specifically illustrating a fixed cellular communications system constructed in accordance with the principles of the present invention; 
     FIG. 2 is a front perspective view of a housing for the components of an interface unit of the type preferably used in the fixed cellular communications system depicted in FIG. 1; 
     FIG. 3 is a front view of the housing depicted in FIG. 2, wherein the front panel has been opened to show the components included within its interior; 
     FIG. 4 is a block diagram of the components included within the interface unit depicted in FIG. 3; 
     FIG. 5 is another block diagram illustrating a fixed cellular communications system constructed in accordance with the principles of the present invention; 
     FIG. 6 is a flowchart diagram illustrating the functions performed by the disconnect supervision/call termination control circuitry included within the fixed cellular communications system of the present invention; and 
     FIG. 7 is a circuit schematic diagram illustrating analog electronic components that can be included within the disconnect supervision/call termination control circuitry of the interface unit depicted in block diagram form in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates a mobile telephone switching office (MTSO)  20 , which in accordance with conventional design, includes a mobile telephone switch (MTX)  22  and cells  24 , each having a radio frequency (RF) antenna  25  associated therewith. MTSO  20  is interconnected with a public switched telephone network (PSTN)  28  to allow for the routing of telephone calls to and from the PSTN. MTSO  20  is further interfaced with mobile telephones  30  and portable telephones  32 , and communication occurs through cells  24 . 
     As further shown in FIG. 1, a fixed cellular communications system  34  includes a private branch exchange (PBX) (or a key system unit (KSU))  36  having a plurality of analog trunk lines  38  connected thereto. The trunk lines  38  are connected, in turn, to interface units  40 . Each trunk line  38  has an interface unit  40  associated therewith. Fixed cellular communications system  34  further includes a plurality of key sets (i.e., business telephones)  41 , which are connected to PBX/KSU  36 . 
     In operation, telephone calls are placed from key sets  41  by depressing a line key to activate the analog trunk line  38  associated with that line key. Thereafter, a dial tone is emulated by the control circuit included within the interface unit  40  associated with the activated trunk line  38 . Upon recognition of the emulated dial tone, the telephone user then dials the desired telephone number and the call is placed in accordance with the protocol of the selected interface unit  40 . 
     As will be appreciated, the placed call is made from a cellular transceiver included as part of the interface unit  40  (see FIG.  3 ). The call is routed through the MTSO  20 . In the case where the telephone call is made to a mobile telephone  30  or a portable telephone  32 , the call is made on a mobile-to-mobile basis. As a result, if both the called and calling parties subscribe to cellular service offered by the same mobile telephone service provider, all switching would only need to be performed by that service provider. This could result in substantial savings for the customers, and the mobile service provider could introduce and market corresponding billing plans. Most importantly, in this case, costs associated with switching by a different mobile service provider and/or a landline service provider would not be incurred, as those providers would not need to route the placed call. It will be appreciated that, referring to FIG. 1, the switching offices for different mobile service providers and for landline service providers are included as part of the PSTN  28 . 
     When a call is placed to a “fixed employee” whose office uses a fixed cellular communications system  34  for its telephone service, one of the analog trunks  38  is activated in accordance with the applicable protocol (e.g., hunt-and-seek). The call is received at the cellular transceiver included as part of the interface unit  40  associated with that activated trunk. If the call has been placed by a mobile or portable telephone user who subscribes to the same mobile service provider as the firm that utilizes the fixed cellular communications system  34 , then the call need only be routed through the mobile switching office  20  for that service provider, and is made on a mobile-to-mobile basis. Again, savings are then possible. 
     FIG. 2 illustrates a preferred housing  42  for the interface unit  40  of the type used in the fixed cellular communications system  34  depicted in FIG.  1 . Housing  42  is preferably, though not necessarily, constructed in metal. As shown, the housing  42  includes a front panel  44 , a rear panel  46  (see FIG.  3 ), a side panel  48  and a top panel  50 . Not shown but also included as part of the housing  42  are a side panel opposite side panel  48  and a bottom panel. 
     Referring to FIG. 3, the front panel  44  of housing  42  is shown in its open position. Preferably, front panel  44  is pivotally connected to one of the side panels of the housing, most preferably by one or more hinges. As shown, the interface unit  40  preferably includes a cellular transceiver  52  for transmitting and receiving cellular telephone calls. Interface unit  40  further includes a circuit board  54  on which control circuitry, including disconnection supervision/call termination control circuitry, is mounted. Unit  40  also includes a data adapter  56  of the type found in prior art fixed cellular communications systems. Data adapter  56  houses control circuitry that emulates standard telephone service (POTS) for cellular transceiver  52  and further emulates control signals for the transceiver. It will be appreciated that the control circuitry mounted on circuit board  54  could be included as part of the control circuitry housed within data adapter  56  and share a common housing therewith. The interface unit  40  further includes a power supply  58 , a programming handset  60  and a modular telephone jack  62 . 
     Cellular transceiver  52  is preferably mounted to the rear panel  46  of housing  42  with the aid of amounting bracket  64  fastened to that panel. The cellular transceiver  52  includes a radio frequency antenna  66  mounted thereto, which antenna extends through a port  68  included within the top panel  50  of housing  42  to facilitate transmission and reception of cellular telephone calls through one of the cells  24  operating at the mobile telephone switching office  20  (FIG.  1 ). 
     Cellular transceiver  52  is electrically coupled to the control circuitry mounted on circuit board  54  and the control circuitry housed within data adapter  56 . The control circuitry is electrically coupled to the modular telephone jack  62 , which is designed to receive a particular one of the trunk lines  38  included within the fixed cellular communications system  34  depicted in FIG.  1 . In this configuration, the control circuitry interfaces between the cellular transceiver  52  and an analog trunk line  38  for the fixed cellular communications system  34 . 
     Power supply  58  is electrically coupled to the control circuitry mounted on circuit board  54 , to the control circuitry housed within data adapter  56 , and to the cellular transceiver  52 . So coupled, power supply  58  serves as a source of electrical energy for all control circuitry and the circuitry included in cellular transceiver  52 . 
     Programming handset  60  is electrically coupled to the cellular transceiver  52  and allows for a variety of functions to be programmed for use with the interface unit  40  generally, and the transceiver  52  specifically. One example of such programming is that the telephone number associated with cellular transceiver  52  can be changed through use of programming handset. When not being used, programming handset  60  is preferably held in place by a mounting bracket  69  mounted to side panel  48  of housing  42 . 
     Referring to FIG. 4, a schematic diagram for interface unit  40  is shown. As shown, interface unit  40  includes transceiver  52 , which receives and transmits RF signals through antenna  66 . Control circuit  70  housed within data adapter  56  is connected to transceiver  52 , at one side, and to the analog trunk line  38  associated with interface unit  40 , at the other side. In that regard, the control circuitry preferably includes a trunk driver circuit. Disconnect supervision/call termination control circuitry  71 , which is mounted on circuit board  54 , is also connected to transceiver  52  and trunk line  38 , as shown. Power supply  58  is connected to transceiver  52 , control circuit  70  and disconnect supervision/call termination control circuitry  71 , as necessary, to provide a source of electrical energy for carrying out the functions of those circuits. 
     An important aspect of this invention is the incorporation of the disconnect supervision/call termination circuitry  71  shown in FIG.  4 . Referring to FIG. 5, the interface unit  40 , which includes that circuitry, is shown (much like in FIG. 1) as being part of a fixed cellular communications system  34  used in a telecommunication network. Private branch exchange (PBX)  36  has a plurality of key sets  41  connected thereto. PBX  36  is connected to a plurality of analog trunk lines  38  and each such trunk line has an associated interface unit  40  connected thereto. The transceiver  52  included within each interface unit  40  is capable of transmitting and receiving radio frequency signals through antenna  66  of the interface unit to an antenna  25  associated with the mobile telephone switching office  20  of a mobile telephone service provider. For calls placed to or from mobile and/or portable telephones  30 ,  32  (see FIG. 1) having users who subscribe to the same service subscriber as the firm controlling the fixed cellular communications system  34 , calls need only be routed through mobile telephone switching offices  20  under the control of the mobile telephone service provider. As a result, the cost associated with placing and/or receiving such calls will typically be less than when an outside provider&#39;s office must be utilized to route the call (be it a central telephone office or a mobile telephone switching office controlled by a different provider). 
     The operation and functionality of the disconnect supervision/call termination circuitry  71  is represented in the flowchart diagram illustrated in FIG.  6 . When a call is terminated by a remote cellular telephone (such as mobile telephone  30  and/or portable telephone  32  shown in FIG.  1 ), an appropriate end-of-call (EOC) signal is generated by that remote telephone and is transmitted to switching office  20 . Switching office  20  receives the EOC signal and, in response thereto, transmits a call end control signal to transceiver  52 . The call end control signal may be, but need not be, identical to the EOC signal. 
     In response to its receipt of the call end control signal, the transceiver  52 , which was activated and therefore previously drawing relatively high current and consuming relatively high energy, “powers down” to its idle state. It will be appreciated that cellular transceiver  52  consumes more energy during a call, as opposed to during any given time interval while it is idling. 
     The disconnect supervision/call termination circuitry  71  constantly monitors the current drawn and/or the energy consumed by transceiver  52 . When the absolute value associated with the monitored one of those physical quantities moves from a state wherein it is greater than an idling “high” threshold value to a state wherein it is less than the idling “high” threshold value for a predetermined period of time, and for so long as it remains less than an idling “low” threshold value for that predetermined period of time, the disconnect supervision/call termination circuitry  71  senses this transition and causes the previously activated/seized trunk line  38  to be released. In effect, once the absolute value of the selected physical quantity moves into and remains within an idling “window” for a predetermined period of time, the trunk line  38  associated with the interface unit  40  is caused to be released by the disconnect supervision/call termination circuitry  71 . In its preferred form, the aforementioned period of time is approximately five hundred milliseconds (500 ms). 
     In particular, referring to FIG. 6, block  100  represents the state wherein the cellular transceiver is idling. Block  102  is a step wherein the disconnect supervision/call termination circuitry  71  determines whether the physical quantity being monitored (e.g., current and/or power) is greater than the idling “high” threshold value. If not, it is determined that the transceiver is idling, or is powered off. If so, it is recognized that the transceiver is in operation. 
     At block  104 , the disconnect supervision/call termination circuitry  71  determines whether the physical quantity being monitored has shifted such that it is now less than the idling “high” threshold value. If not, operation loops back to block  104  and the disconnect supervision/call termination circuitry  71  awaits a transition of the physical quantity being monitored from a condition where it is greater than the idling “high” threshold value to a condition where it is less than that idling “high” threshold value. 
     Once this transition occurs, it is then determined, at block  106 , whether the clock has already been triggered. If not, the clock is triggered, as represented by block  108 . In any event, at block  110 , the disconnect supervision/call termination circuitry  71  then determines whether the physical quantity being monitored is greater than the idling “low” threshold value. If not, the clock is reset at block  112  and the process reverts back to the operation depicted by block  104 . 
     If the disconnect supervision/call termination circuitry  71  determines that the physical quantity being monitored is greater than idling “low” threshold value, then operation continues to block  114  and it is determined whether the clock has reached its predetermined value such that the physical quantity being monitored has stayed within the applicable idling “window” for such predetermined period of time. If not, operation reverts back to the operation depicted by block  104 . If so, the disconnect supervision/call termination circuitry  71  generates a control signal to release the trunk line, as shown by block  116 . The process is then resumed, as shown by block  118 . 
     It will be appreciated that the process illustrated in FIG. 6 is merely illustrative of the functions that are performed by the disconnect supervision/call termination circuitry  71 . 
     It will be appreciated by those skilled in the art that each transceiver  52  has its own corresponding idling level. Accordingly, the idling level of the physical quantity selected to be monitored will need to be measured before the interface unit is put in use. A conventional meter can be used for that purpose. The “high” and “low” threshold levels that bound the threshold “window” can then be set so that the disconnect supervision/call termination circuitry  71  is calibrated and functions properly for the transceiver in which it is used in conjunction. 
     It will be appreciated that the disconnect supervision/call termination circuitry  71  will not be needed to release its associated trunk line  38  when the call is terminated by a fixed cellular communications system user (e.g., a user of one of the key sets  41  shown in FIG.  5 ). Upon termination of the call at a key set  41 , the active trunk line  38  is released and, in response thereto, the control circuit  70  causes the transceiver  52  to generate and transmit an EOC signal to the MTSO  20 . Thereafter, the transceiver “powers down” to its idling state. In response to the received EOC signal from transceiver  52 , the MTSO  20  signals the remote telephone (be it landline or cellular) to disconnect so that appropriate call tear down occurs at that remote telephone. 
     Referring to FIG. 7, basic analog circuit components that can be utilized to perform the preferred functions of disconnect supervision/call termination circuitry  71  are shown. The disconnect supervision/call termination circuitry  71  includes a window comparator  72 , preferably including two operational amplifiers  74 ,  76 , and a one-shot device  78 . The noninverting input terminal  80  of operational amplifier  74  is set to a voltage level that corresponds to the high end boundary of the idling threshold window for the physical quantity being monitored. It will be appreciated that this voltage level is not necessarily, and preferably is not, equal to the high end boundary of the defined idling threshold window. It will also be appreciated that this voltage level can preferably be altered for use with different transceivers, which, in turn, are likely to have different idling conditions. To effectuate such alteration, a voltage ladder network having a variable resistor could be used. It will be appreciated that other manners in which to effectuate such alteration could also be used. 
     The inverting terminal  82  of operational amplifier  74  is preferably electrically coupled to the transceiver  52 . The output  84  of amplifier  74  is electrically coupled to the noninverting terminal  86  of operational amplifier  76 . The inverting terminal  88  of operational amplifier  76  is set to a voltage level that corresponds to the low end boundary of the idling threshold window for the physical quantity being monitored. It will be appreciated that this voltage level is not necessarily, and preferably is not, equal to the low end boundary of the defined idling threshold window. Also, it will be appreciated that this voltage level can preferably be altered. 
     The output  90  of operational amplifier  76  is electrically coupled to the input terminal  92  of one-shot device  78  and the output terminal  94  of the one-shot device is electrically coupled to the tip terminal (T) of the trunk line  38  associated with the interface unit  40 . 
     In operation, during a call, transceiver  52  is drawing sufficiently high current and/or consuming sufficiently high energy such that the voltage level present at the inverting terminal  82  of operational amplifier  74  exceeds the voltage level present at the noninverting input terminal  80  of that op amp. As a result, operational amplifier  74  is disabled, which causes the voltage level at its output terminal  84  (and at the noninverting input terminal  86  of operational amplifier  76 ) to be driven LOW. This renders operational amplifier  76  inactive also. 
     Upon termination of the call by a remote telephone, the transceiver, in response to a signal received from the MTSO  20  (FIG.  1 ), “powers down” and the voltage level at the inverting terminal  82  of operational amplifier  74  falls below the voltage level present at the noninverting input terminal  80  of that op amp. Accordingly, operational amplifier  74  is enabled and the voltage level present at its output terminal  84  is driven HIGH. This voltage level, which is also present at the noninverting input terminal  86  of operational amplifier  76 , exceeds the voltage level present at the inverting input terminal  88  of that op amp, causing that op amp to be enabled. As a result, the input terminal  92  of one shot device  78  is pulsed with a voltage signal. If the voltage signal present at the input terminal  92  of one shot device  78  remains HIGH for a sufficiently long period of time, the output terminal  94  of that device goes HIGH and “lengthens” that pulse so that trunk line  38 , to which it is electrically coupled, is released. It will be appreciated that an electronic latch or similar device might also be included to aid in the performance of these functions. 
     As will be appreciated, the idling “window” is set by the voltage levels present at the inputs of the operational amplifiers  74 ,  76  of window comparator  72 . It will also be appreciated that the timing is set by the characteristics of one shot device  78 . 
     Those skilled in the art will also appreciate that the disconnect supervision/call termination circuitry  71  can alternatively be embodied in a variety of other manners, including with digital electronic circuit components and/or a microprocessor programmed to perform the aforementioned functions of the disconnect supervision/call termination circuit. 
     While this invention has been described with reference to an illustrative embodiment, it will be understood that this description shall not be construed in a limiting sense. Rather, various changes and modifications can be made to the illustrative embodiment without departing from the true spirit and scope of the invention, as defined by the following claims. Furthermore, it will be appreciated that any such changes and modifications would be recognized by those skilled in the art as an equivalent to one element or more of the following claims, and shall be covered by such claims to the fullest extent permitted by law.