Hi sharonfromtracy,
Thanks for contacting Apple Support Communities for assistance today. As I am understanding, RTT is turned on and you are unable to switch it off in Accessibility Settings.
It appears that your iPhone 6 is still running iOS 9. Our latest article on RTT is for iOS 11.2 and above: Make and receive RTT calls on your iPhone.
Do you have any Restrictions active that could be causing the switches to be inaccessible for you? You can check in Settings > General > Restrictions.
You can also try updating to iOS 12 where you would find the RTT settings in Settings > General > Accessibility > RTT/TTY.
Be sure to back up before updating: How to back up your iPhone, iPad, and iPod touch.
Check out how to update here: Update your iPhone, iPad, or iPod touch.
Let me know how that goes.
Have a great day!