im having the same problem, i went from contract to payg as i had ordered a new phone from another network, i phoned and switched of my answer phone then it magically switched itself back on, i dot intend on putting any credit on it just to switch it back off!!!
its annoying, as those people that have forgotten my no has changed keep phoning and leaving messages!!!!
i think its bad practice to charge payg customers for voicemail and customer services.
i will keep an eye on this thread, hopefully somebody will know