How can an FM-phone signal be produced in a transmitter?

Producing an FM-phone signal in a transmitter involves feeding the audio directly to the oscillator. In frequency modulation (FM), the frequency of the RF carrier signal is varied in accordance with the instantaneous amplitude of the audio signal. When audio is fed directly to the oscillator, it effectively alters the frequency of the generated RF signal based on the inputs of the audio wave, creating the desired FM modulation.

The other methods mentioned do not accurately apply to conventional FM transmission. For example, directly modulating the RF signal in a simplistic dual-tone approach does not create the complex frequency variations needed for FM. Additionally, complex modulation schemes often refer to advanced techniques like digital modulation, which are not a straightforward way to generate an FM analog signal. Pulse-width modulation, while a valid method of encoding information, does not produce frequency modulation directly. Thus, feeding audio directly to the oscillator is indeed the most accurate and effective way to achieve FM modulation in traditional applications.