Nikon MOTO QA4 - Manual (Page 17)

15 get started
TTY does not work from mobile phone to
mobile phone.
1 Open the main menu and touch 7 Settings > Calling >
Advanced Call Settings.
2 Touch TTY Mode, then select a TTY setting.
voice privacy
You can encrypt outgoing voice calls and use
secure channels when available.
1 Open the main menu and touch 7 Settings > Calling >
Advanced Call Settings.
2 At Voice Privacy, slide the switch to On.