Someone's fleecing me, but I don't know who

To compound a bad evening (yes, Milan were excellent tonight), I checked my mobile phone credits. Being on "pay-as-you-go" (or "Prepaid" auf deutsch), I'd topped up my credits back up to 15 euro just this last weekend. It's the 78th minute and Gilardino is just about to hit the final nail in United's 2006/2007 Champions League coffin when I check my account and find that I only have "5 euro 10 cents" left in credits despite the fact that I haven't used my phone at all since I topped the credits up to 15 euro. This is the final confirmation to me that someone's been nicking my credits, however that's possible.

I'd smelt something fishy around three months ago, but wasn't sure whether I was going mad and that texting to the UK from Switzerland with a German mobile phone number was the real reason behind the strange loss of mobile credits. Now I know that this isn't the case, and that I'm going to have to either get a new "pay-as-you-go" deal or sign up for a mobile phone contract. Either way, I'm going to have to do something.