As is well detailed in Dr. Meyer's book, "Old Time Telephones!" (Chapter 19, page 199) the circuitry of the original 500 set was quite different from production 1951 or so onward. Current flow regulation (which was intended to balance transmission characteristics regardless of the length of line) was provided by an optional "ballast" lamp. located in a little package called an "equalizer". Although this type of regulation was soon scrapped, British manufacturers used the same type of regulation in their "700" series sets, and this too, was an option. Most manufacturers eventually duplicated the 500 (Northern Electric, because of Bell Canada's Bell System affiliation, was first). One company that never copied the 500 was Automatic Electric. In early years, AECo used a potentiometer to regulate current flow in their type 80 (and this idea was experimented with by the Bell labs in their 1947/1948 prototypes). Eventually, a more modern network was introduced, which used varistors to control the amount of current flowing through the loop.

The 2500 set, a Touch-Tone version of the 500, was introduced c. 1965. Originally produced in a 10 button version, two buttons were soon added for end-to-end signaling. Originally designated as the Sextile and Octothorpe, these buttons soon became known as the (often hated) "Star" and "Pound" keys. They introduced many of us to a computer world!