1760 should work in most countries.
If it isn't doing, you could try dialling +44 7802 090 100 which should result in your being sent an SMS with the correct number to dial from your location to access your voicemail (you'll need to know your pin) - although I'm not sure whether there's an option for you to disable it from the IVR options?
Alternatively, assuming you're a Pay Monthly customer, you could download TuGo as I'm sure there's an option within the app to disable VM?
I work for O2, however any advice or opinion given in this community should be considered my own and not necessarily representative of Telefónica UK Limited.
