35% of people had a telephone. It would be rare for poorer people to have a telephone so it would be more likely for a wealthy person to have a telephone. Later in the years, more people got a telephone. This also means "poorer people" had a phone as well. This shows that not only wealthy people had a phone.
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I can only speak from personal experience as a person who grew up in the 1970's in a middle class Canadian setting.
Almost everyone in the middle class (and therefore certainly in the upper class) had a phone. And I suspect that most "poor" people had a phone as well.
Of course, there were no cell phones at that time (or at least they were virtually unknown among the masses).