From the manual in a 2 second google search:
Changing the TTY Baud Rate
By default, your phone?s TTY baud rate is set to
45.45, the baud rate required for TTY calls within
the U.S. To make calls outside the U.S., set your
TTY baud rate to 50.0.
To change the TTY baud rate:
1 From the main menu, select Settings > Phone
Calls > TTY > Baud.
2 Select the baud rate for your location.