Do you have the R4ADMIN program? You should be able to get into it that way and straighten things out.
You can dial into the main voicemail number from your terminal running the above software and put ,,,993 after the phone number (you might have to adjust the number of commas so it sends the 993 after the system answers).
Otherwise just connect directly to the serial port on the front of it.
I think you can also administer it from the 999 mailbox, but I didn't have to do that very often.