How do you hear people over the phone?

The microphone (mouthpiece) of a telephone turns the sound vibrations of your voice into electrical or electronic signals, which are transmitted through the various phone lines and data links to the phone at the other end, which has a speaker (earpiece) that turns the signals back into sound. Newer phones use digital technology, whereas older phones use a simpler analog conversion, which is still sensitive enough to fairly well recreate the frequencies of your particular voice at the other end of the conversation.