I've just left Vodafone to give GiffGaff a try (give it go and say scienceboy recommended you). The voicemail is a different number (111), but I couldn't find any information on the phone's help system to this forum about changing it on an N97. I finally found on the phone after trawling the phone's menus...
Setting > calling > call mailbox > options > change number
What a work up. If there are actually any N97 still out there, I hope this helps.
If i have helped at all a click on the white star below would be nice thanks.