Possible hijacked vodafone mailbox

I have a vodafone pre-pay phone. I recently left it at home and when i called it to from a landline to check my messages using the 087 5*** **** deally and put in my PIN, it kept telling me i was entering it incorrectly.

This might mot be too alarming of itself, but i remember ringing into my mobile's mailbox before from a landline and getting through immediately to the main mailbox menu, e.g. immediately after dialling and before entering my password, i heard the message: "you have two new messages..." etc and remarking to myself that someone could easily change my password if they inadvertently got into my mailbox this way, while I had the phone set to divert.

I am wondering if this is what has happened, and kindof suspect someone has changed my mailbox password so they can listen to my messages (possible ex trouble): is there any way of finding out?
Has anyone had trouble like this before?

Just dial 171 from your mobile and change the password. Yes, many people don't change the default '0000' password and many people listen in on other peoples messages.

It is very common that people don't change the default code and its the route of endless jokes in our office.