Real nice finding.

Not that it really annoyed me, but I could clear up some older voicemail numbers

You do not need to edit the file directly, but may also use gconftool.
To read:
To write:
But maybe file editing is easier
BUT remember to reboot phone immediately or do
gconftool --shutdown; killall gconfd-2; gconftool --spawn
(as otherwise the editing may get overwritten by another change to that gconf directory by another app)

@pichlo
Did you also go to phonebook/settings/voicemail-number and selected the right (current) number and saved? Else you will always see the last used voicemail-number.