Since you have an SC1000 console I am making the assumption you have an SX-200 or an SX-200 ICP system. Also assumed is that you have the Express Messenger voicemail. You can determine this by looking at the bays and see if one of the cards says Express Messenger. With these assumptions in mind:
Look through any papers or docs left behind by the owner for passcodes. You will need to get into mailbox 999 (or 9999) to change the greetings. If there is a voicemail button on a phone, press that first, then hit star until she asks for the mailbox number. Then input 9's. At that point you will need to put in the admin, manager, or technician passcode.
There may be a passcode that was left at default, you can PM me as I cannot post those in an open forum.
To change call routing, you really will need an installer's assistance with either a laptop plugged into the system, or by dialing into the onsite modem (if any). Even to get the modem info documented, I believe an onsite visit may be required.