You didn't memorize ten random digits. Area code, exchange, four digit extension is six things...

Area code was likely all the same, maybe 2-3 tops, and associated both with the following exchange and geography, and used extensively.

Exchange, also likely all the same, maybe 6 at most, and again likely associated with geography.

You were an extreme outlier if you bothered to memorize dozens of ten digit phone numbers in the era before everyone had a cellphone.

The average person doesn't even have ten good friends, much less a need to memorize dozens of phone numbers. They would buy address books / contact books to write down dozens of numbers, not memorize numbers they very rarely use.

I remember 2 out of the 3 landline numbers (one is my parents, still using that) I used to know, my 2 ICQ numbers, and my mobile phone number (which is still the same now, 20 years later). I never felt the need to remember a ton of numbers, that’s what phone books (digital or analog) are for and why I now use a password manager ;)

Sure but for most of them it was only ~ the last 5 digits that differed. Unless you were more cosmopolitan and made lots of long distance calls.