I was under the impression that Phoney, as popularised by JD Salinger's Catcher in the Rye, was referencing the manner of voice people tend to adopt when on the phone. There is certainly a correllation between usage and the spread of the telephone, but I am no etymologist.
Telephones and phoninessEdit
The first attested usage of "phoney" in print is reportedly 1899, by George Ade. Bell called his device a telephone (appropriating a word that dates back to 1835 as a French word for a signalling device) from about 1876. The problem is that the artificial-manner etymology should be a widely mentioned one if it might be the origin. The term seems to have its origins in social classes that could not afford telephones in 1902. It was not especially used in reference to telephones. A "fawney" (or "fawny") was probably used in the manner of the inexpensive replica designer watches that can be bought on the streets of many cities around the world.