Telephone line security system

Claim 1

A telephone line security system which controls the telephone numbers which may be called on a telephone line, comprising: a telephone line interface circuit which is connected to the telephone line and provides electrical isolation between the telephone line and the telephone line security system; a dial tone detection circuit which is connected to the telephone line interface circuit that provides an output signal when a dial tone is detected on the telephone line; a digit determination circuit which is connected to the telephone line interface circuit and provides an output unique to each digit of a telephone number asserted onto the telephone line; a secure digit sequence programming circuit which is subscriber programmable and stores each sequence of digits occurring in telephone numbers which are to be prohibited; an authorization digit sequence programming circuit which is subscriber programmable and stores each sequence of digits which are selected to be code number sequences; an exception digit sequence programming circuit which is subscriber programmable and stores each sequence of digits which occur in telephone numbers which are selected to be exceptions to the sequences of digits occurring in telephone numbers which are to be prohibited; a sequence determination circuit which produces an output upon receipt of each input from the digit determination circuit, which output indicates the number of the input within a sequence of the inputs; a system reset circuit which produces an output that resets a telephone call authorization circuit upon initial application of power to the telephone line security system; a secure digit sequence detection circuit which produces an output if the sequence of digits input from the digit determination circuit coincides with a sequence of digits stored in the secure digit sequence programming circuit; an authorization digit sequence detection circuit which produces an output if the sequence of digits input from the digit determination circuit coincides with a sequence of digits stored in the authorization digit sequence programming circuit; an exception digit sequence detection circuit which produces an output if the sequence of digits input from the digit determination circuit coincides with a sequence of digits stored in the exception digit sequence programming circuit; said telephone call authorization circuit is connected to the telephone line interface circuit and asserts an output onto the telephone line which makes communication impossible if the secure digit sequence determination circuit has an output, if the exception digit sequence determination circuit does not have an output, and the telephone call placed immediately prior was not of a sequence of digits which caused the authorization digit sequence determination circuit to produce an output.