Whilst I agree mostly that people tended to learn more phone numbers before mobiles, the fact that they tended to be shorter must have helped considerably. In the UK, my landline number was a mere 5 digits (for those living in the same town as me - ie. where all my friends came from; you'd need to know the area code if you lived in another town though). Now, with mobiles, numbers are all 11 digits so a lot harder to memorise.