Welcome to Android Central! Which phone? RTT is Real Time Text, which can be very important for the hearing impaired.
On a Samsung phone, open your Phone app, tap Menu>Settings>Real Time Text>RTT Call Button, and change it to Visible During Calls (instead of Always Visible). On other phones, it might be in the main system settings under Accessibility. I don't think you can completely get rid of it. It should only show up if you're calling someone whose phone supports RTT or is using a TTY device.