AT&T has finally introduced a much-awaited feature which could potentially change the way its subscribers communicate - Wi-Fi calling.

The network operator pushed out the feature for certain smartphones on Thursday, Oct. 8, two days after receiving the green signal from FCC as reported by us.

So now, AT&T subscribers who own the iPhone 6s, iPhone 6, iPhone 6 Plus and iPhone 6s Plus will be able to receive and make calls by deploying a Wi-Fi network. The only caveat is that the above-mentioned Apple smartphones need to be on iOS 9.

Interestingly, rival carriers Sprint and T-Mobile both offer Wi-Fi calling for some devices. T-Mobile has been offering the service since 2007 despite not having the FCC permit AT&T received. The question then arises - why did AT&T take so long to bring on board Wi-Fi calling?

Unlike rival carriers who were offering the Wi-Fi calling feature sans a waiver, AT&T had to wait for the FCC permit to come through. The network operator has made assertions that the FCC is playing favorites and intentionally ignored the violation of rules from T-Mobile and Sprint, calling the episode "asymmetric regulation."

AT&T's SVP of external and legislative affairs Jim Cicconi summed up the carrier's sentiments aptly in a blog post.

"We are left scratching our heads as to why the FCC still seems intent on excusing the behavior of T-Mobile and Sprint, who have been offering these services without a waiver for quite some time. Instead of initiating enforcement action against them, or at least opening an investigation, the agency has effectively invited them to now apply for similar waivers and implied that their prior flaunting of FCC rules will be ignored," wrote Cicconi in the blog post.

Irrespective, now with AT&T supporting Wi-Fi calling, subscribers to the carrier's network will be able to take advantage of the feature. Wi-Fi calling is handy when one is an area with no coverage. In such situations, one can use the Wi-Fi network available in a restaurant or café to make a call. Since Wi-Fi calling is built into a device, one does not need to download any third-party app.

AT&T's Wi-Fi calling feature will only be available to users when texting or making a call from Puerto Rico, the U.S. and U.S. Virgin Islands. By contrast, T-Mobile lets its subscribers make Wi-Fi calls pretty much anywhere if one has Wi-Fi access.

For those looking to enable Wi-Fi calling on their eligible iPhone, navigate to Settings> Phone> Wi-Fi Calling. The set-up process is a tad lengthy so one needs to be patient.