Unable to disable RTT (Real-Time Text) in calls

Expected Behavior

Phone calls are made as normal, with RTT available as an option in accessibility settings.

Current Behavior

Phone calls are always made in RTT mode despite RTT being disabled (according to adb shell settings get secure rtt_calling_mode) and there is no option to turn it on or off in accessibility settings.

This is an issue because RTT breaks some automated systems, disables the proximity sensor making calls much easier to drop, and the UI in the Lineage dialer is also broken in RTT mode making the end call and ... buttons invisible in light mode.

This only started happening after updating from Lineage OS 20 to 21 and switching to Verizon, I did both at the same time so I don't know which actually caused it.

Steps to Reproduce

https://files.catbox.moe/2fp87f.mp4

  1. Place call
  2. Wait about 3 seconds
  3. Hear audible tone and dialer switches to RTT mode

Device information

/device xz2c /version lineage-21 /date 2024-06-16 /kernel 4.9.227-perf+ #1 Fri May 31 13:38:56 UTC 2024 /baseband 845-sdm845.genprodQ-00052-48 /mods Google Apps

I have read the directions.