There's a mic laying on the floor and AT&T is headed for the exit. The U.S.' second largest wireless carrier just called out the Federal Communications Commission and dropped a dime on a pair of rivals, all regarding its efforts to deliver Wi-Fi calling to its customers.

About a year ago, Apple introduced the iPhone 6 series and the ability for the handset to move seamlessly between cellular networks and Wi-Fi while on a call. T-Mobile, and later Sprint, rushed  to adopt the new feature, Wi-Fi calling, but now it's been revealed why AT&T didn't appear to embrace the technology with similar enthusiasm.

In a letter to FCC Chairman Tom Wheeler, AT&T's James Cicconi, senior VP of External and Legislative affairs, accuses T-Mobile and Sprint of adopting Wi-Fi calling without taking the necessary step of delivering TTY (Teletypewriter) or obtaining the necessary waivers from the FCC to forego the inclusion of transcription feature for the deaf.

TTY is notoriously unreliable on IP-based networks, which is what Wi-Fi calling relies on, stated AT&T in the letter. So the wireless carrier, ranked two in the U.S., was faced with a conundrum.

"From a business perspective, therefore, we were faced with a Hobson's choice: enter the market without a waiver, as our competitors had done, or approach the Commission with a technology solution that would provide an alternative to TTY technology for IP-based Wi-Fi calls and a request for a temporary waiver of the Commission's TTY requirements until that technology could be implemented," says AT&T (PDF).

AT&T chose option number two: create a TTY alternative that's suitable for IP-based network, while obtaining a waiver from the FFC until that tech was ready. Well, the FCC didn't respond in a timely fashion and, a year later, AT&T couldn't release Wi-Fi calling on its intended launch date, Sept. 25, because it has yet to secure that waiver.

Despite its efforts to proffer filings with the disability community and the telecom industry, the FCC staff didn't expedite a public notice for AT&T's filing and, on top of that, the commission subjected the waiver request to a 45-day period for commentary.

"I do not reference this delay to denigrate the staff's efforts here, but simply to observe that the processes for keeping pace with technology transitions at the Commission are slow, particularly here where coordination amongst three bureaus/departments is involved."

AT&T had been trying to coordinate the launch of its Wi-Fi calling with the release of iOS 9. It has been working with RTT (Real Time Text) as an alternative to TTY with the hopes of having Wi-Fi calling meet the FCC's standards.