Apparently, via other forums, if your caller has RTT on, you get it automatically (unconfirmed).
My Galaxy S10+ has NO off switch for RTT. The default dialer via Verizon/Samsung took a UI dive recently.
I installed the Google dialer and the UI was better, but RTT was on EVERY call.
I had to get RID of it, and finally, I found ACR Phone Dialer in the Google Play Store: