I don’t know that the “alphabetical order” theory really jives.
Yes, English speakers read from top-left to bottom-right. But if folks can use a calculator with the numbers running from left-to-right, bottom-to-top, then I can’t see why any one would be confused by corresponding letters doing the same. (That is, 2=ABC, 3=DEF, just like today.)
Same goes for the placement of the zero. If the phone numbers were arranged like a calculator, the zero could still remain at the bottom… just like a calculator already does.
A usability study makes sense to a point. But they’re often suspect. Fig. 3 shows six grouped arrangements. Group I, item I-C was shown to have “significantly shorter keying time” as well as a “significantly lower error rate”... but they’re push buttons arranged similar to a rotary phone (the gap twisted about 90 degrees clockwise and numbers reversed order)!!!
Anyway, not to be cynical. This is an interesting Q. I just not convinced we nailed the answer perfectly yet.
It does make sense that Bell Labs would have a more than casual influence over telephone push button design.
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Interesting info about mechanical calculators.