RUT9xx supports USSD commands.
Open RUT9 WebUI, navigate to Services -> CLI and connect to it.
To send USSD command use 'gsmctl -U "<command>"' (gsmctl -U "*124#"). If command is sent successfully gsmctl will return "OK".
You can read response with command "cat /tmp/ussd".
Please note that response contains the value of last command and it changes if you use USSD command agai or could potentially remain the same if operator returns error to this command.