The problem with rewarding only the first inventor, or the first to file the invention, is that this is a completely arbitrary designation that is fundamentally unfair to the other independent inventors. It also illustrates problems of obviousness; if four distinct inventors could all arrive at the same invention, independently, within months or even a few short years of each other, doesn't that imply that the invention is 'obvious to one skilled in the art'?
The independent invention defense fixes this.
And of course, the telephone isn't the only example. We've talked in the past about the airplane, the automobile, and the digital computer. Add telephones into the mix and you have four of the most important inventions of our time, all independently invented by more than one person. Yet we award a monopoly over the invention to only one of the discoverers. This is fair, how?