Apart from the release of the iOS 9, another reason why AT&T subscribers were so anxiously excited for the arrival of the firmware is the Wi-Fi Calling feature for iPhones. However, the carrier says it needs to delay the rollout a little longer as it still needs to wait for approval from the Federal Communications Commission.

To date, only T-Mobile and Sprint offer the Wi-Fi calling. Verizon and AT&T have yet to offer this functionality to customers on their networks.

The Wi-Fi Calling feature is be beneficial for users who happen to be in a place with poor cellular signal but with a stable Wi-Fi network.

In August, we reported that AT&T enabled the Wi-Fi Calling feature on iOS 9 beta. The report also stated that only a limited number of areas supported the feature. Also, it only worked with the iPhone 6 and iPhone 6 Plus.

During the time, AT&T announced the feature would roll out soon. Many customers expected that it would come along with the launching of iOS 9.

However, due to pending approval from the FCC, the carrier said it needed to delay the official unveiling of the new feature.

On the other hand, the company did say that users who had beta-tested iOS 9 before the firmware was released will still be able to use the feature.

"AT&T tested Wi-Fi Calling through the iOS 9 beta and we are prepared to support commercial launch of the service once approved by the FCC," AT&T stated.

AT&T also mentioned that it is also waiting for the FCC's approval for specific features dedicated for hearing impaired users. The Wi-Fi Calling feature, according to reports, does not use the TTY (Text Telephone) technology. It utilizes instead the RTT (Real-Time Text) system, which still needs approval from the FCC.

Meanwhile, AT&T has yet to announce whether this feature will also be rolled out to devices other than the iPhone in the near future.