“Our sound varies while communicating through a telephone. Why?”
In telephonic communication (as also in any other form of communication), the sound/voice is converted into electrical signals which are then modulated for transmission to distant locations. The receiver reconstructs the original message through a reverse process. During these stages, the quality of the sound can be changed. All the wave forms may not exactly be present in the reproduced voice and hence the sound seems different.
Better faithful reproduction can be achieved by various techniques