The 2500 was another great Donald Genaro design from the Henry Dreyfuss Associates design firm. Mr. Genaro had a dominant role in the redesign of the 500 for the new touchtone dial and even was involved in the difficult decision of how the keypad would be set up (at the time adding machines had a "bottoms up" keyboard, and so a great deal of thought and testing went into the decision to have the number 1 in the upper left hand corner). Mr. Genaro also designed nearly every ATT telephone from the 2500, Trimline (that was his baby) and so on until the last ATT telephone made in the mid-90's. We'll have more on his work later (check back for a link). This enduring design has persisted from the mid-60's to the present day. The very first touchtone sets originally had no * or # keys (this was the 1500 set) but in the late 60's the design was changed to allow more functionality. All of my sets have 12 buttons (I do not stock the early, 10 button models!)
Trivia: the # key was named the "octothorpe" to honor Olympian Jim Thorpe.