Bell Labs had been doing research and trying to come up with a push button replacement for the rotary dial since the 1940's. Initial prototypes had buttons that plucked two reeds together to generate the dual tones. With the invention of the transistor at Bell Labs in 1947, however, the idea of plucked reeds gave way to electronic oscillators.
After lots and lots of prototypes and field trials, the first production model 1500 was introduced to the public in 1964. It had a 10-button touch tone dial. In 1968, the 1500 was replaced with the 2500. The model 2500 was virtually identical to the 1500, but replaced the 10-button pads with 12-button pads that added the '*' (star) and '#' (what Bell Labs originally wished to name the octothorpe, though this name never stuck).
The 1500/2500 models were basically just like the original model 500, except that they had touch tone dials instead of a rotary dials, and a slightly different housing.
The housing on the 1500/2500 was designed by Donald Genaro of Henry Dreyfuss Associates. Although several prototypes had been made using the 500 housing, and although many 500's in the field would later get converted by the use of a circular adapter plate around a touch tone dial in the original 500 housing, I guess Donald Genaro just felt the 500 body wasn't good enough, and that he had to re-invent the wheel, so he could claim credit for something.
As with the model 500, other manufacturers, under various licensing arrangements, also made 1500/2500 sets nearly identical to those made by Western Electric.
I have more information to offer here on the history and production changes of the various brands of 2500's:
For more information on the various model numbers in the 1500/2500 family, please visit this page on Paul Fassbender's site.
For more information on development and timeline photos of the 1500/2500 touch tone models, please refer to this page on Faul Fassbender's site.
For more information on COLORS, visit this page on Paul Fassbender's site.
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