Subject says it all…
The question came up with the wife today, when did we go from describing telephone numbers from KL5 (first 2 as letters) to 555 (first 2 as numbers).
I’d imagine it was in the 60s or so, but we weren’t sure.
-Butler
IIRC, it was in 1967, at least in my little bit of Southern California. I remember having my parents try to explain why our phone number was now 889-xxxx, and not TUrner9-xxxx. we moved the next year, and had a new number of course, which is why I remember the year.
And that sounds about right… I told my wife that somebody on SD would know, and right quick too!
Thanks,
-Butler
This thread covered the topic.
Remembering ads for PLumber3-1919 from my youth. 
Actually, in some cities, you can occasionally find the alphabetical phone numbers in phone-book display advertising and on old signage. I remember a lot of this in Los Angeles.
Robin, who knows where the old (213) 889 prefix is. 
The first phone number I dialed as a toddler (377-2411 -a switchboard at Emory, where my dad worked then) was in 1965, and I immediately became hyperaware of phone numbers as todlers tend to do with anything new. They were 7-digits in 404, and “exchange letters were rare”. They used 7-digits in 1967 in LA, too, but I saw far more of the old Exchange letters in LA
[I thought of them as ‘ole people’ relics, but now I realize the prevalence of ‘old style’ numbers was because Angelinos had adopted the phone earlier and more widely than Atlantans by the switchover, and had more habits to break. Modernity is a paradox]
Until now I never realized I’d even been alive in the “exchange era”. I’ll go sit in my rocking chair and sip ‘rheumatiz medicine’.