This interesting circuit found in American documentation generates a busy signal, without you having to take the phone off the hook if you do not want to answer. The components used are common.
This circuit was found in documentation from the 70s. We can try current JFET transistors....
This is the power supply circuit for the amplifier in the previous circuit. The source is...
This bridge circuit is designed for 8-ohm speakers delivering a few watts of power. The circuit is from...