If you see this symbol in the status bar, it means that TTY is on.
Teletype (TTY) machines are used by deaf and hard of hearing people to communicate by typing and reading text. If you have the iPhone TTY Adapter, available at www.apple.com/store, you can use iPhone with a TTY machine.
Choose Settings > Phone to turn TTY on or off.
Inserting a SIM into an unlocked iPhone from a carrier not found on this list enables TTY.