Go to 'setup.cmd' from the Dialler menu. Press enter for the number to keep the default. Next type your net id so that it will be used by default and press enter. Next type the new init string and press enter. Exit and restart Trumpet.
Press esc at the prompts and esc again to abort the script.
Go to 'setup.cmd' from the Dialler menu. Type the proper prefix and separate it from the default number with a comma and press enter. Next type your net id so that it will be used by default and press enter. Next press enter for the init string to keep the default. Exit and restart Trumpet.