I had same problem a couple of years ago when on holiday,
I was getting voice mail messages and this is why you are charged, turning off voicemail
Will stop cold callers from making connection, unfortunately family wouldn't be able to leave a message though.
I got on to vodaphone when they billed me an extra £47, originally they said they couldn't help
Other than advising to turn off voice mail, but then I told them I was terminating my contract immediately, amazingly they then paid me back the £47 and paid a £15 good will gesture payment as well, so worth trying that with whoever your with.