Dear xyz,
Fascinating question...
This may be the editor's column you remember:
http://musicman.net/EID_Comment-Winter02.pdf
However, the Chapanis work I was referring to wasn't about the number of digits in a phone number, it was about the layout of the keypad.
I have no information, other than the apocryphal anecdotes about Miller's "magic number 7" to explain why telephone numbers are 7 digits long in the U.S.A. (though I suspect the total population of phones in different areas might have something to do with it as well).
I saw several references to some working memory brain theories that affirm, if not explain the convention:
http://prl.aps.org/abstract/PRL/v103/i21/e218101
However, another reference quotes Miller himself as saying:
And finally, what about the magical number seven? What about the seven wonders of the world, the seven seas, the seven deadly sins, the seven daughters of Atlas in the Pleiades, the seven ages of man, the seven levels of hell, the seven primary colors, the seven notes of the musical scale, and the seven days of the week? What about the seven-point rating scale, the seven categories for absolute judgment, the seven objects in the span of attention, and the seven digits in the span of immediate memory? For the present I propose to withhold judgment. Perhaps there is something deep and profound behind all these sevens, something just calling out for us to discover it. But I suspect that it is only a pernicious, Pythagorean coincidence.
It gets messy when you start talking about "chunking". For instance, I don't think I remember phone numbers near my home as 10 digits. When I hear an area code or an exchange that I recognize, introspection tells me that I remember that as a single entity -- I actually sometimes visualize an area (or a mobile coverage map).
Even without chunking, I think the grouping itself is useful (nnn-nnn-nnnn) as a "performance cue". I find I can more easily transcribe a teleconference pin if the calendar host puts in the artificial hyphen (and if the number is 7 digits long -- if it's 8 digits then I start stressing about overseas phone number patterns ;-).
Perhaps it is because I'm a vocal performer, but I find it very comforting to slot arbitrary numbers into the rhythmic pattern neh-neh-NEH neh-NEH-neh-NEH. This is a 4-beat phrase of 7 eighth notes and a trailing whole note with a familiar "polka" emphasis on all but the first primary quarter note beat. (Though for music, I much prefer the jazz-style syncopations.)
