If you see the TTY icon, which looks like a telephone receiver over a series of white dots , on the status bar at the top of your iPhone, the TTY feature is turned on. This feature, which is designed so hearing-impaired and speech-impaired people can conduct phone calls, can be turned off in the iPhone Settings.
Tap on the Settings icon on the home screen.
Scroll down the Settings screen and tap Phone.
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Find the TTY button and slide it to the Off Position.
TTY stands for
TTY devices can be used both on landlines and on cell phones -- analog or digital. An adapter connects the iPhone to a TTY device.