His most important contributions were to the theory and implementation of the use of loading coils and the first wave filters designed to what was to become known as the image method.

He developed a method for transmitting analog telephony over much greater distances than had previously been possible by the insertion of loading coils into the line at carefully calculated intervals to increase the inductance.

[5] Despite the rather arcane legal arguments surrounding this, it is unquestionable that Campbell was the first to actually construct a telephone circuit using loading coils.

Campbell was tasked by AT&T to investigate the possibility of improving line quality with the use of iron-copper bimetallic cable invented by John S. Stone,[8] another AT&T engineer.

However, Campbell was struggling to set up a practical demonstration over a real telephone route with the budget he had been allocated.

[10] The first telephone cable using loaded lines put into public service was between Jamaica Plain and West Newton just outside of Boston on May 18, 1900.

When considering whether to allow Campbell to go ahead with the demonstration, their engineers had estimated that they stood to save $700,000 (equivalent to $21,700,000 in 2023) in new installation costs in New York and New Jersey alone.

This behavior, and the lumped-element networks being used to create artificial lines for test purposes, suggested to Campbell a possible topology for a filter with similar characteristics.

The ability to send multiple conversations over the same wires resulted in very substantial savings in cable installation costs.

The modulation system used (single-sideband suppressed-carrier transmission) and the ITU standard remained the primary method of telephone service distribution until it began to be supplanted by digital techniques from the 1980s onwards.