Apple's iPhone accounted for 78% of AT&T smartphone activations in Q1 2012

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AT&T sold 5.5 million smartphones last quarter, and 4.3 million of those were Apple's iPhone, the company revealed in its earnings report on Tuesday.

Sales of Apple's iPhone accounted for more than 78 percent of smartphones sold at AT&T. The carrier said iPhone sales were "helped by AT&T's 4G network, which lets iPhone 4S download three-times faster than other U.S. carriers' networks."

Among the 4.3 million iPhones that AT&T sold in the first quarter of calendar 2012, 21 percent of those were new to AT&T. The total 5.5 million smartphones AT&T sold in the three-month period was a new record for the company.

AT&T's results compare to 3.2 million iPhones activated at rival carrier Verizon. The iPhone did not carry as large of a share at Verizon, though it still represented more than half of the 6.3 million smartphones sold during the quarter.

AT&T said on Tuesday that about 30 percent of its existing postpaid smartphone subscribers are on 4G-capable devices. Starting with the release of iOS 5.1, the iPhone 4S began advertising to users that they are on a "4G" network when utilizing AT&T's high-speed HSDPA network.

The change allowed AT&T to further differentiate itself from competitors Verizon and Sprint, which offer slower CDMA networks that cannot reach HSDPA speeds. However, "true" 4G remains exclusive to long-term evolution devices, such as Apple's third-generation iPad, which is the company's first LTE-capable product. AT&T made no mention of iPad sales in Tuesday's press release.

The new iPad comes in two different models that allow connectivity with Verizon and AT&T's respective LTE networks. The third-generation tablet also differentiates between AT&T's "4G" HSDPA and true LTE connectivity by displaying "LTE" on the device when connected to a high-speed 4G LTE network.

For the first quarter of calendar 2012, AT&T earned consolidated revenues of $31.8 billion, up $575 million, or 1.8 percent, compared to the same period a year ago. The carrier earned 60 cents diluted earnings per share, compared to 57 cents diluted EPS in the first quarter of 2011.