Sorry to hear of the problems with your voicemail whilst abroad, we appreciate you taking the time to raise this via the forums and welcome to the community.
Can you confirm if Roaming was set up before you left the UK? Do you have access to other services like calls, texts and data whilst away? Where abouts are you?
We do have some help here and if you have access to texts we could send you a message to help resolve the issue and reset the PIN.