During most of the period in which memorizing phone numbers was common, most people weren't memorizing 10 digit numbers. Rather, local calls to your neighbors

During most of the period in which memorizing phone numbers was common, most people weren't memorizing 10 digit numbers. Rather, local calls to your neighbors required you only remember 4 digits when you all shared the same three digit prefix. And if you didn't share a prefix but shared an area code, you only had to memorize 7 digits. For the calls most people were making most of the time, memorizing 4 was typical and 7 was occasional. This did begin to change before the rise of phone number storage tech, but it doubtlessly spurred the adoption of that tech.