This project has been specially designed to introduce you to SMD (Surface Mount Devices). Surface Mount is really not new. It started as far back as 1940 with a hybrid circuit in a digital watch. A chip was cemented onto a slab of ceramic material having gold tracks and film resistors. The LED display was mounted in a similar fashion and the whole assembly coated in epoxy to protect it before fitting into the watch case.source

Here is a simple telephone ring tone generator circuit designed using only a few components. It produces simulated telephone ring tone and needs only DC voltage (4.5V DC to 12V DC). One may use this circuit in ordinary intercom or phone-type intercom. The sound is quite loud when this circuit is operated on +12V DC power supply.However, the volume of ring sound is adjustable.source

The telephone ring generator shown below generates the needed high voltage from a simple switching mode power supply (SMPS) which employs a CMOS Schmitt Trigger square wave oscillator, 10 mH inductor, high voltage switching transistor (TIP47 or other high voltage, 1 amp transistor) and a driver transistor (2N3053). The inductor should have a low DC resistance of 1.5 ohms or less. The switching supply must have a load connected to prevent the voltage from rising too high, so a 22K resistor is used across the output which limits the voltage to about 120 DC with the phone ringer disconnected and about 90 volts DC connected. source

The circuit requires four astable multivibrators for its working. Therefore two 556 ICs are used here. The IC 556 contains two timers (similar to 555 ICs) in a single package. One can also assemble this circuit using four separate 555 ICs. The first multivibrator produces a rectangular waveform with 1-second ‘low’ duration and 2-second ‘high’ duration. This waveform is used to control the next multivibrator that produces another rectangular waveform.source

The third amplifier in the LT1491, which is configured as a lowpass filter, converts the square wave output of the oscillator to a sine wave by filtering out unwanted harmonics.Finally, the 87VRMS and the – 48VDC parts are handled by the fourth amplifier in the LT1491 and its steering of two external 15V regulators.source