Using phonebooks

Another reason why we had to memorize phone numbers was because of phonebooks, also known as phone directories. 

These bulky and heavy books contained the contact information of households and businesses that were customers of phone companies. Every year, each phone company would release a phonebook with all the contacts, which were delivered to their clients annually.

You could easily look up any contact and find people you knew in these books. However, due to their substantial size, it was very inconvenient to carry them around. So, memorizing a few important contacts was very useful. It was also common to carry your own small phonebook with a few important contacts written by hand.