Calls to voicemail outside Europe zone are charged as per a voice call. Please note that outside of Zone 1 - Europe, you'll also be charged the incoming rate when someone leaves you a voicemail.
Please see the full roaming guide in Mobile pricing for all roaming charges.
If you absolutely want to avoid charges while abroad, there's nothing to stop you leaving the SIM card at home. Then the mobile just becomes a mini computer, but will work for anything that can use local wifi, @ian_alex1.
