He is credited with synthesizing the designs of several existing printers into his line of devices[1] which became the dominant apparatus for automated reception and transmission of telegraph messages.

By this time his machinist skills had expanded into areas involving work in light machinery, paper sorting machines, and safe locks.

Western Union purchased American Telegraph Company following the Civil War in 1866 and positioned Phelps as superintendent of the mechanical department, first in Troy, later in Williamsburg and finally in New York City.

Phelps continued to work on printing telegraph systems for many years with full support from Western Union management, who sought any competitive advantage.

In the mid-1870s, Western Union's president William Orton assigned Phelps to experiment with the harmonic telegraph in hopes of extended it to a working telephone.