It's true that the cost of voice services has fallen over the past decade, and even in the past year: Sprint and T-Mobile both offered attractive rates, so Verizon and AT&T slashed the prices of their unlimited calling plans by $30 in early 2010.
But what that doesn't show is how much people are actually spending on their cell phone bills. For AT&T customers under contract, spending has gone up over the past four years. AT&T's average revenue per user for postpaid customers has increased