If you are busy and cannot answer or do not wish to answer your phone, this circuit will give a busy signal without you
having to leave the phone off the hook. After a predetermined time, the circuit is deactivated. U1 forms an astable multivibrator that can be set for a time up to 10 minutes by values of R3 and C3. When SI A is depressed, U1 starts, and Ql latches, which powers the circuit. At the end of a time interval determined by R3 and C3, Q2 and Q3 cut off and remove power from the circuit. During the operation, S3 throws a 150- resistor across the phone line, which simulates an off-hook condition.