I've copied this from a similar query on another board (thanks, drey_p).
This procedure should apply to all handsets on the Vodafone network. Enter the figures on your dialler:
To set: enter **61* then enter the number you want to divert your calls to - in this case 121 for voicemail - followed by *10* the number of seconds you want your phone to ring for – followed by # (e.g. **61*121*10*30# ) then press SEND.