AT&T just reported its Q3 2012 earnings, and once again the iPhone was a big part of the company's portfolio. The second-largest carrier in the US reported 4.7 million iPhone activations — though, as usual, it didn't differentiate between sales of new devices and activations of any used or unlocked phones that may have signed up for AT&T's network. As for the total smartphone picture, AT&T sold 6.1 million phones, trailing the 6.8 million smartphones Verizon moved last quarter. While AT&T says that it had a "record sales quarter" for Android and Windows Phone devices, it's pretty clear that the iPhone is a bigger part of the picture at AT&T than it is at Verizon.

Overall, the carrier managed to add 678,000 net new customers in Q3, more than double what it added in Q2 — this puts the company at a total of 105.9 million subscribers, up from 100.7 million a year ago. Smartphones make up 63.8 percent of the company's postpaid subscriber base, helping to increase data revenues by 18.3 percent. Overall, this helped AT&T take in $3.6 billion in profit for the quarter, flat year-over-year.

AT&T also shared some information on its LTE buildout, saying that its network now reaches more than 135 million US residents, and that its "4G" network covers more than 285 million residents. As for the carrier's new Mobile Share data plans, AT&T saw "nearly two million sales" in the first five weeks, with more than a third of those signing up activating plans with more than 10GB of data per month. AT&T's holding an investor call this morning at 9AM ET, and we expect to hear a lot more about its new data plans and LTE network then — we'll update this post with any new information.

Update: AT&T CEO Ralph de la Vega just said that "about twice that of our closest competitor" — with Verizon activating about 651,000 iPhone 5 devices, it seems AT&T sold in excess of one million devices. He also said that "we face significant supply contraints on the iPhone 5," so it seems that Apple's new handset will still be a bit challenging to find this fall. He also specifically mentioned Nokia and its Lumia line of smartphones when talking about the need to find less expensive phones with lower subsidies, saying, "we like the lineup that we have with Lumia devices coming in from Nokia — they're very nice devices and they fit that same mold."