What three aspects of the electrical signals carried over the telephone network can be varied by modems to encode data in the signal?

Modems can vary the amplitude, frequency, and phase of the electrical signals transmitted over the telephone network to encode data. Amplitude modulation changes the strength of the signal to represent different data values, frequency modulation alters the signal's frequency for encoding, and phase modulation shifts the phase of the signal to convey information. By manipulating these aspects, modems effectively translate digital data into a format suitable for transmission over analog telephone lines.