At the heart of the issue is compatibility of non-hearing customers that use “text telephony” (teletypwriters), which does not reliably work on Wi-Fi based communications. AT&T had been waiting for a response to a request to implement Wi-Fi calling in support of an alternative protocol for the deaf, called RTT (real-time text), which is under development.

AT&T had been planning to launch Wi-Fi calling in September, but the nod from the FCC was not forthcoming, pushing the carrier’s plans back to some undetermined time. Yesterday, the FCC gave AT&T the needed waiver so that it does not have to guarantee TTY transmissions over Wi-Fi calls.

According to the FCC, there are “major technical barriers” that get in the way of reliable transmissions of TTY over IP networks. This is as much a limitation of TTY which was invented in the 1960s. However, current regulations require that phone companies and wireless carriers support TTY, hence the need for a waiver.

AT&T is pushing the FCC to levy some type of enforcement action against T-Mobile and Sprint since they have been operating outside the regulations for “quite some time.”

source: FierceWireless