Today, I searched all over the interwebs to find data that tracked cell phone ownership over time in the U.S. - I found the amazing World Bank Data Bank and graph tool and made this:
Unfortunately, the "lower middle income" legend label is missing (see second line from bottom). Interestingly, you can see a slight decrease in the U.S. overall subscriptions per 100 people from 2013-2014, and a leveling off of subscriptions for lower middle income americans in this same period. You can also see what's probably the proliferation of the "work phone" beginning in 2007, when the number of subscriptions among high-income americans went above 1 per person so that in 2013 there are 121 cell phone subscriptions per 100 people in this category.