Telephone security device

A telephone security device has a code signal recognition means for storing two sets of variables, one of which governs the current operating mode of the system, and the other of which comprises first and second predetermined multiple-digit numerical code numbers responsive to proper inputs for producing first and second output signals, respectively. These output signals control a mode register means which has two stable states. In one state, the mode register means responds to the first output signals of the code signal recognition means to enter into its other state, thereby rendering the inhibiter means inoperative and allowing any number of outgoing calls to be made. The mode register means remains in this other state even following disconnection of a subsequently made call. However, the mode register means responds to the second output signal of the code signal recognition means to re-enter its one state, in which case the inhibiter means prevents the successful completion of certain calls as determined by the current operating mode of the system. The code signal recognition means includes an electrical alterable read-only memory ("EAROM") which may be continually reprogrammed without requiring a disassembly of the telephone set.

This invention relates to telephone security devices. 
The specification of my U.S. Pat. No. 4,099,033 discloses a telephone 
security device for preventing unauthorised use of a telephone instrument, 
the device comprising: 
controllable inhibit means for preventing the successful setting up of a 
connection in response to operation of the called number selecting means 
of the instrument; 
code signal recognition means which has an input for receiving code signals 
and which is operable for responding to the reception thereby of a code 
signal corresponding to a predetermined code signal to produce a 
corresponding mode output signal, and 
mode register means which has two stable mode states, which is connected to 
said code signal recognition means and said inhibit means, and which is 
operable when it is in one of said two states to respond to the reception 
from the code signal recognition means of said mode output signal to enter 
the other of said two states and to remain in the said other state even 
following disconnection of a subsequently made telephone call, the mode 
register means being further operable to control said inhibit means so 
that, for as long as the mode register means is in said one of said two 
states, the inhibit means prevents the successful setting up of a 
connection in response to operation of said called number selecting means 
to make outgoing calls to at least some destination telephone numbers and, 
for as long as the mode register means is in said other state, the inhibit 
means is inoperative and hence allows any number of outgoing calls to be 
made normally. Such a telephone security device will be referred to herein 
as "A telephone security device as defined." 
According to the present invention there is provided a telephone security 
device as defined in which the code signal recognition means comprises, 
for storing a code signal to which the code signal recognition means is to 
respond and a signal representative of an operating mode into which the 
device has been set by a previously received code signal, an electrically 
alterable read-only memory. 
The characteristics of such devices, known as EAROM devices, are such that 
data can be stored in the devices and will remain stored even if the 
energy supply to the memory is removed and then reapplied, typical data 
retention periods being of the order of tens of years. This data can, if 
required, be erased and new data stored by the application of the 
appropriate signals. The details of the EAROM devices are well known to 
those skilled in the art, and commercial devices currently available are 
easily able to offer the storage capacity required for both the code 
signals and the operating mode signals, and can be read repeatedly without 
affecting the stored contents. One application requiring similar 
characteristics where these devices are currently used is in programmable 
personal calculators.

FIG. 1 shows a modification of the telephone security device particularly 
described in the specification of U.S. Pat. No. 4,099,033. In the figure, 
the input device 5 may be the telephone dial contacts and these are 
connected to a code generator 6. The latter includes a timing circuit A6 
and a pulse counter A7. The timing generator circuit A6 produces the 
necessary timing signals for the TSD. The pulse counter A7 counts the 
dialled impulses from the input device 5 to form a binary number 
representing the value of the last dialled digit and presents it to a 
shift register A13. The store 8 comprises a non-volatile memory and its 
various control circuits. The non-volatile memory is embodied in an 
electrically alterable read-only (EAROM) A9, having an address control A8 
and a Read/Write Control A10. The EAROM A9 is configured in a suitable 
arrangement for storing the operating parameters, the address control A8 
and Read/Write Control A10 providing the necessary signals to access the 
EAROM A9. The comparator 7 comprises a Buffer A11, a comparator circuit 
A12 and a shift register A13. The Buffer A11 holds the comparison code 
number supplied by the EAROM A9, the shift register A13 holds the dialled 
number and the comparator circuit compares these two numbers. The control 
circuit 9 uses the result of the comparison and the current operating mode 
information from the EAROM A9 to INHIBIT or to allow the call. The control 
circuit 9 also updates the operating mode held in the EAROM A9 when the 
comparator 7 indicates that this is necessary. The switch 10 INHIBITS or 
allows the call as instructed by the control circuit 9. 
When an EAROM is used as the storage means in any particular embodiment, 
the subscriber will, via mode setting circuitry, only be able to alter the 
mode storage bits, i.e. the mode of the device. The mode codes would be 
set by removing the EAROM, which would be on a plug-in module and 
inserting it into an appropriate socket on a small programming device 
containing a simple circuit which would enable the mode codes to be 
altered to the required numbers. Such a programming device, an example of 
which is described hereinafter with reference to FIG. 2, could be the 
property of the Telephone Authority and would be in the possession of 
qualified personnel. This operation would require the dismantling of the 
telephone before the EAROM was removed for reprogramming and would thus 
normally be performed by qualified personnel. While this would not prevent 
the possibility of fraudulent alteration of the mode codes, such action 
would require specialist knowledge and equipment. 
Many push-button and card-dialling type telephones operate on the principle 
that the digits of the number being dialled are first entered into and 
stored in the telephone, the appropriate dialling pulses or tones being 
generated either during or after entry of the number being dialled. The 
circuitry used to implement this function is normally in the form of an 
integrated circuit, (known colloquially as a dialling chip), and in at 
least one commercial form, has the capacity to store up to twenty digits 
before generating dial pulses or tones. Thus one use of my invention would 
be to include the principle thereof in the integrated circuit used to 
perform the dialling function. This would allow the comparator and control 
circuit to monitor the digits as they were being stored in the integrated 
circuit and the decision to INHIBIT the call could then be made before any 
dialling signals were sent to the exchange. This would have the benefit of 
preventing the exchange from being loaded by any dialling signals 
generated before the INHIBIT decision was made as is the case where the 
digits are monitored as they go to line. In this implementation the 
inhibition of the cell would consist, in the simplest case, of the 
dialling signals just not being generated. 
The technical implementation of my invention in such an integrated circuit 
is relatively simple, its exact form being dependent upon the other 
functions implemented in the said integrated circuit. As a result this 
embodiment will not be described in any detail. 
My invention requires that two sets of variables be stored and be easily 
readable in any integrated circuit implementing the principles of the 
invention, these variables being: 
(1) The current operating mode of the device (Inhibit, Normal or Local). 
(2) The mode setting code numbers to which the device is to respond. 
(1) above will normally be altered by the authorised subscriber as part of 
the device's normal operation. 
(2) above will generally only be altered by qualified personnel from the 
appropriate Telephone Authority using a programming device. 
In the event that any other facility available to the subscriber requires 
the dialling of the subscriber's own number but it is required that the 
telephone does not enter the INHIBIT mode as a result, the INHIBIT code 
can be altered to any number that does not compromise an existing 
subscriber's number or an existing service number. One possibility would 
be to use one of the unused three-digit numbers reserved by the Telephone 
Authority. 
The use of a three-digit code as the INHIBIT code instead of the 
subscriber's own number would have the advantage that when the device is 
embodied in a rotary dial type of selecting means only three sets of dial 
pulses would be sent to line before the INHIBIT decision and consequent 
disconnect condition occurred. (As previously stated, such INHIBIT code 
pulses would not be sent to line in a dialling chip type embodiment). 
In any embodiment it would be advantageous for the device to generate, via 
the subscriber's earpiece, a number-unobtainable tone once the call had 
been inhibited, thus dissuading the unauthorised caller from continuing to 
attempt to obtain the number. This could easily be implmented by an 
oscillator circuit which is switched into the telephone circuit once the 
inhibiting relay is activated. Further details of how this feature could 
be implemented will be found in the description of FIG. 3 of the 
accompanying drawings. 
Turning now to FIG. 2 a suitable programming device P for the store 
comprises a keypad A15, a display A14, an EAROM address generator A16 and 
a command decoder A17, and is shown connected to the store. The keypad A15 
is used to preset the mode setting code numbers in the EAROM A9 of the 
store 8 via the EAROM address generator A16 and the EAROM address control 
A8. The mode setting code numbers are displayed by the display A14 as they 
are entered via the numeric portion of the keypad A15. When the complete 
number has been assembled, the appropriate mode button (i.e. LOCAL, 
NORMAL, INHIBIT) is selected. This is detected by the command decoder A17, 
which then causes the numbers to be loaded into the EAROM A9 via the EAROM 
address generator A16 and EAROM address control A8. All three mode setting 
code numbers are selected in this manner. 
The method of programming the TSD store could either be: 
(a) by removing the store module and inserting it into an appropriate 
socket in the programming device, or 
(b) by taking the output from the programming device via a lead and plug 
and plugging the lead into an appropriate socket situated in the telephone 
security device. 
The telephone security device shown in FIG. 3 is intended for connection to 
the line between the subscriber's set and the exchange at any point along 
that line. For convenience, it will usually be sited adjacent the 
subscriber's set or it will form part of the subscriber's set. The 
embodiment is suitable for use with telephones comprising any sort of 
impulse dialling mechanism, e.g. rotary dial mechanisms, push-button and 
card dialling mechanisms, and by making minor changes to the interface 
circuits between the line and the device, it may be adapted for tone 
dialling systems. 
The main difference between this embodiment and my original rotary dial 
embodiment is that it is connected in parallel to rather than in series 
with the telephone line. This removes the need for the device to generate 
dialling pulses. Calls are inhibited and cleared, as before, by the 
disconnect relay 10. 
Referring to FIG. 3, the embodiment comprises a power supply 41 which 
extracts power from the telephone lines 40 during the line breaks 
generated in dialling. During the periods in which the lines 40 are 
short-circuited by the dial switch, the circuit is maintained in an active 
state by the charge stored on a capacitor in the power supply. This 
capacitor is recharged each time a line-break occurs. 
Other possible methods of extracting power from the telephone lines are 
available. One method has the power supply placed in series with the 
telephone line so that power is continually extracted from the telephone 
line (although at a sufficiently low level so as not to overload the 
exchange or interfere with the normal operation of the telephone system). 
Where voltages exceeding those directly available from the telephone lines 
are required (as they often are for many low-power metal oxide 
semiconductor integrated circuits from which the telephone security device 
may be implemented) a D.C.-D.C. converter can be used. Both power supply 
methods could be used. The latter method has the advantage that power is 
continuously available. 
Since this embodiment is connected in parallel to rather than in series 
with the telephone line, a dial pulse detector circuit 42 is included. 
This circuit differentiates between dial pulses and other signals carried 
by the line. Any dial pulses detected are presented to the code generator 
6 in a form similar to that generated by the dial contacts. If this 
embodiment were to be used in a tone dialling system then the dial pulse 
detector 42 would be modified to extract the dialling information from the 
dialling tones. 
Whatever the power supply or dial pulse extraction methods the circuit 
operation is identical to that described and illustrated with reference to 
FIG. 1 above once the dial pulse information has been made available to 
the code generator 6. 
Unlike the embodiment of U.S. Pat. No. 4,099,033, the present embodiment 
does not need to generate dial pulses so no dialling circuit is included. 
A tone generator 43 is included. This circuit, activated by the signal to 
the disconnect relay 10, generates a signal of the same frequency as that 
used to generate the unobtainable-number tone at the exchange. This warns 
the subscriber that the circuit has been activated, (if necessary, to 
uniquely identify the operation of the circuit a different frequency from 
the unobtainable-number tone can be used). 
The device of U.S. Pat. No. 4,099,033, together with the developments and 
additional features thereof disclosed herein, may be embodied as software 
or hardware in a suitably programmed computer, for example in the computer 
which is used in a stored program control (SPC) electronic telephone 
exchange. In a normal system comprising such an exchange, the dialling 
pulses or signals formed at the subscriber's set are received by a 
computer which then acts, in accordance with its stored program, to make 
the desired connection. To embody the present invention in such a system, 
the computer is programmed to receive the dialling pulses or signals and 
then to make the desired connection only if the call is not one which is 
prohibited having regard to the mode, i.e. the aforementioned NORMAL, 
LOCAL or INHIBIT modes which has been registered by the computer in 
respect of the relevant subscriber's set.