The move announced Sunday follows an identical one by Verizon Wireless in April. AT&T's new policy applies to any customer whose contract expires in March 2014 or later.
Extending the time between phone upgrades saves the phone companies money, since they subsidize each new phone by hundreds of dollars to make it available to customers for $199 or less.
The change reflects the growing popularity of expensive phones. Verizon subsidized upgrades after just 13 months until January 2011, just before it introduced the iPhone, one of the most expensive phones on a wholesale basis. Carriers pay Apple over $600 for it.
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