AT&T (NYSE: T) just rolled out Wi-Fi calling to its first Android handset last week, but the operator is already routing more than 4 million calls a day over Wi-Fi.

The nation's second-largest operator first began to offer Wi-Fi calling on newer iPhones last fall, and last week it extended the service to Android users for the first time, announcing support on the LG G4. Bill Smith, AT&T's president of technology operations, told attendees at an investors conference this morning that he expects usage to ramp up in a big way as Wi-Fi calling becomes available on more of AT&T's Android phones.

"We're now carrying over 4 million calls a day on voice over Wi-Fi," Smith said. "We just recently started turning up Android-based operating systems, so I think that's going to go up tremendously, and we're doing that at extremely impressive performance levels."

AT&T hasn't pursued Wi-Fi calling as aggressively as T-Mobile, which first launched the service in 2007 and last year counted 7 million users making calls over Wi-Fi. But Smith said the increasing uptake of Wi-Fi calling bodes well as traffic continues to ramp up, forcing carriers to look to unlicensed spectrum as well as licensed airwaves.

"Unlicensed has to come into the mix when you think about wireless solutions," Smith observed. "About 80 percent of the wireless traffic today is handled over Wi-Fi networks, so we're actually pretty bullish about how we take not only Wi-Fi but other unlicensed bands. And if you build a control plane that is sophisticated enough to manage that effectively, it can really help you…. So when you consider that this has been kind of an un-engineered asset that we've been taking advantage of, I think when you look going forward we will be sophisticated enough to use unlicensed bands more effectively and in a more sophisticated manner."

Meanwhile, Smith said AT&T is deploying its WCS spectrum "on an as-needed basis," and it plans to put the AWS-3 airwaves it won at auction last year to work beginning in 2018. And while some investors have expressed concerns that the operator may be reigning in its capex, Smith said AT&T continues to invest heavily in its network both in the U.S. and as it expands its operations in Mexico.

"You can't look at capex spend on a quarter-by-quarter basis; one data point is not a trend," Smith said, noting that the company's move toward SDN and NFV technologies are already paying dividends. "In '15 (and) '16 we're going to deploy about 200 percent of the capacity that we did in '13 (and) '14, and we're going to do it for 75 percent of the cost."

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