TTY mode is for hearing impaired customers to use their handsets in conjunction with their TTY device. They would have to use the 2.5 mm jack to connect the TTY device with their headset jack after the customer chooses the option of the TTY mode they want to use from phone settings in tools and settings.

The options are:
TTY only-for calls in which you type and read text on your TTY device.

TTY VCO (Voice-Carry-Over) —for calls in which you speak into your phone and read text replies on your TTY device.

TTY HCO (Hearing-Carry-Over) —for calls in which you type text on your TTY device and listen to voice replies on your phone’s speaker.

I hope this helps you!