AT&T customers who use the latest iPhone models and iOS 9 will immediately be able to use WiFi to make and receive calls in the U.S. AT&T Wireless customers who use iPhone 6s or 6 smartphones and iOS 9 will now be able to make and receive calls over WiFi, just two days after the company received a waiver from the Federal Communications Commission for rules that require accommodations for hearing-impaired customers who rely on TTY services. The waiver, which was issued Oct. 6, was required because teletypewriter services don't operate properly over WiFi networks.