The telephone combined the knowledge of how sound works together with a variable electric signal that could be sent over great distances.
Instead of the telegraph's simple electrical pulse - on or off - the telephone uses an electrical signal that changes in amplitude (volume) and frequency (pitch) across time - in other words, a 'continuously varying signal'.
This is more like real life, and means that the telephone is able to carry a human voice. But for the scientists of the day it required a better understanding of electricity as well as technological progress in the areas of cables and switches.