The normal way to turn voicemail on and off is through the appropriate settings menu on your phone. In order to help you with that you need to tell us what phone you have.
---------- Post added at 01:35 ---------- Previous post was at 01:33 ----------
Though if you insist on using the dialling the codes directly, you can find the standard codes for all networks here:
https://www.google.co.uk/search?q=gs...il+divert+code