This circuit allows you to control a device from the telephone. For example:

Connect this device to an unused phone system extension in your home or office. When the phone rings Capacitor C2 will charge until the break over voltage of D3 is reached. This turns on the transistor closing the relay. The relay output could be used as a reset button or a door open control. The number of rings depends on the position of R2. When the capacitor finishes dumping to the transistor the relay opens, resetting the circuit.