Welcome to the Community Forums! I'm terribly sorry for our delayed response.
In order to access your mobile voicemails from abroad, we do ask that you set up voicemail and PIN before you go. However if you are still away, let us know and we'll be able to reset your voicemail PIN for you over PM so you can gain access to your voicemails again.
You can find more information on what checks to make if you're going abroad here