Every TTY Profile has its own service/additional service indicator that enables the appropriate selection of Bearer Capabilities (BC) and Low-Layer Compatibility elements for the SETUP message. These indicators are used to filter incoming calls and to tell the remote site which kind of connection the user wants to establish for outgoing calls.
To change these default values for outgoing calls, use the AT commands AT+iS (specify service/add service indicator) or AT+iU (specify BC/LLC values). See AT-Command Set for more information on the AT commands.