Question

How does the telephone transform sound energy to electrical current?

Answer 1

It can be converted with use of a transducer.
A transducer is a device which converts one form of energy into another form of energy. The conversion can be to and from any type of energies like electrical, mechanical, photo-voltaic, electromagnetic etc..

Examples of transducers
Motors - ConvertsElectric energy into mechanical energy
Microphone - Sound vibrations into electric energy

It oper the same way the microphone in your telephone does. Your speech is converted to electrical impulses, which are then converted at the other end by the transducer in the receiver's earpiece.