Free stuff?

So I was just training the new girl on the extremely tedious task that I'm currently undertaking, and my phone buzzed twice to say I had new messages. I checked them, and they told me that my Post Office payment of $20.00 would be processed in the next five minutes. I am obviously at work and not at the Post Office, so I was a little baffled. I have just checked my balance, and it seems that someone has mistakenly/generously topped up my phone account for me.

If the former, too bad for them for not getting their own phone number right, they deserve something like this. If the latter, a huge thank you to my anonymous benefactor. In the slim chance of the latter, I'm not going to inform Vodafone and try to give the money back. Is that wrong of me?