Fortune 500 rank: 7
2009 profit: $12.5 billionAT&T's exclusive contract for Apple's iPhone has driven new subscribers and earnings in recent years.
But all those new devices strained its network in 2009. The nation's largest phone company had to spend more than $17 billion on improvements to its system.
Offering big subsidies on the iPhone to attract customers also hurt earnings, which slipped in 2009 as AT&T added 7 million new wireless subscribers.
NEXT: 9. Wells Fargo