When the doorbell is pushed, you'll hear a low tone that will "slide up" to a higher frequency. The frequency of the AF oscillator is determined by coupling capacitor, C1 and the value of the resistance connected between the base of Q1 and ground. That resistance, RBG is equal to (R1 + R2) R3. First, assume that S1 is closed and R2 has been adjusted to produce a pleasant, low-frequency tone. Capacitor C3 will charge through R6 until it reaches such a voltage that it will cause diode D1 to conduct. When that happens, the value of RBG is paralleled by R4. Thus, because the total resistance RBG decrease, the output tone slides up in frequency. Capacitor C3 will continue to charge until the voltage across D2 and D3 causes those diodes to conduct. Then RBG is paralleled also by R5, the total resistance again decreases, and the oscillator's frequency again increases.