iphone 5 visual voicemail uk

Hi there, I've just gone onto Orange with an iPhone 5 and can't get the visual voicemail to work at all. It says I need to set up a password and greeting but then when I've done all that and hit 'save' it comes up with a dialog box saying 'Voicemail Unavailable. Unable to connect. Try again later' or 'Voicemail Unavailable. Access not authorised' (or something). I've been trying on and off for several days now with no luck. Anybody got any good ideas? Thanks