Voicemail from abroad

[Chickydoo]

Crossing fingers that someone might be able to help me.
I am abroad at the mo' & a work voicemail has come in that I really need to listen to.
I have an iPhone 6 on O2.
I can't access my voicemails, apparently I need a special PIN...I don't have one, & have no idea where to begin.
Any ideas would be greatly appreciated.
TIA