What does it mean when RTT pops up on a call with only one of my contacts

Make or receive RTT/TTY phone calls · Open the Phone app. · Select your contact and tap their phone number. · Select RTT/TTY or RTT/TTY Relay.

Answer or make RTT or TTY calls from your iPhone

Set up RTT and TTY · Go to Settings > Accessibility. · Tap RTT/TTY or TTY, then do any of the following: If your iPhone has Dual SIM, choose a line. Turn on ...

Set up and use RTT and TTY on iPhone