Wondering why you can't use Wi-Fi calling on your new AT&T iPhone while Sprint and T-Mobile customers have no problem? To hear AT&T tell it, that's because its two competitors went ahead and implemented the feature without getting proper clearance from the FCC.

In a letter to FCC chairman Tom Wheeler seen by The Verge, AT&T's legal SVP James Cicconi says that T-Mobile and Sprint started offering Wi-Fi calling services despite the commission not having granted a waiver for TTY (teletypewriter) support; Cicconi says AT&T faced a "Hobson's choice" of either following its competitors' lead or developing a technical alternative while asking the FCC for a temporary waiver. TTY can be less reliable on certain Wi-Fi networks.

"...Our competitors provide those services in defiance of the commission's rules."

AT&T chose the latter, and earned what Cicconi calls "unanimous" support for its RTT (real-time text) solution from the industry and the disability community. AT&T filed for a temporary waiver in June so that the FCC would have time to amend its rules to permit RTT as a replacement for TTY ahead of iOS 9's launch last month, but it hasn't yet been granted. "Because the commission has not granted AT&T's waiver petition," Cicconi writes, "we are not in a position to provide Wi-Fi calling services to our customers even while our competitors provide those services in defiance of the commission's rules."

Cicconi speaks of a "growing concern at AT&T that there is an asymmetry in the application of federal regulations to AT&T on the one hand and its marketplace competitors on the other hand," saying that the Wi-Fi calling issue "simply adds fuel to that fire." Cicconi asks for AT&T's waiver request to be granted "without further delay."

When asked for comment on the matter by The Verge's Chris Ziegler, this was T-Mobile CEO John Legere's response: