Phone numbers, at least in the US, were much simpler 15+ years ago. Most people had a number starting with an area-code + first-3, out of say, 5-10 possible combinations.

EG, 123-444, 123-323, 123-789, 555-121. So, you really needed to remember the prefix and the last 4. There were exceptions, but most of your circle lived within this finite space of telephone numbers.