AT&T has opened up FaceTime over a cellular connection to nearly all iPhone users, including those on tiered data plans. 

Mark Collins, senior vice president of data and voice products for AT&T, said in a blog post that the move is the "result of ongoing testing," as to the effect of FaceTime on the company's network.  

"We’re announcing AT&T will enable FaceTime over Cellular at no extra charge for customers with any tiered data plan using a compatible iOS device," Collins wrote. 

This means iPhone 4S customers with tiered plans will be able to make FaceTime calls over the AT&T cellular network. AT&T had previously made FaceTime over cellular available to customers with a Mobile Share plan and those with an LTE device on tiered plans. 

FaceTime over Wi-Fi remains available for all customers who have a compatible iPhone or iPad.

Collins said AT&T has already begun updating its systems and processes and expects to start rolling the update out to customers on an ongoing basis beginning in the next couple of weeks. Customers do not need to do anything, as the update will be applied automatically over the next few months. 

Admitting FaceTime onto its cellular network has been a slow process for AT&T. At first the company said it would only allow users who switched to the company's new shared data plans to access the app on its wireless network, and other customers’ FaceTime use was limited to Wi-Fi. 

Next it said it would allow any customer with an LTE-capable iOS device on a tiered or shared data plan to use Apple's FaceTime video conferencing app over a cellular connection.

Advocacy groups, including Free Press and Public Knowledge, threatened to file a complaint with the FCC over AT&T's FaceTime policy. The groups argued that by excluding some customers from using the app, AT&T was in violation of net neutrality laws.

Jim Cicconi, AT&T senior executive vice president of external and legislative affairs, acknowledged "concerns" over the policy in a November 2012 blog post on the company's website, saying that the initial decision to limit the app was due to concerns over the sheer number of iPhones on AT&T's network.

"AT&T has by far more iPhones on our network than any other carrier," Cicconi wrote, adding that when Apple rolls out new changes, as it did with iOS 6, "it can have a much greater, and more immediate, impact on AT&T’s network than is the case with carriers who have far fewer iPhone users."

Cicconi said that with the FaceTime app already preloaded on tens of millions of iPhones, there was no way for AT&T's engineers to model usage and assess network impact.

"It is for this reason that we took a more cautious approach toward the app," Cicconi wrote. "To do otherwise might have risked an adverse impact on the services our customers expect – voice quality in particular – if usage of FaceTime exceeded expectations."