Android :: Losing Credit On Pay As You Go

I'm not a great mobile phone user and until recently used to get by with an old Nokia pay-as-you go. My iPod Touch recently got stolen, so I was looking for a replacement and thought if I got myself an Android phone (although I had no experience with Android), I could still listen to music and even have a camera, whilst having less in my pocket.

So I bought myself a Vodaphone 845 pay-as-you-go running Android 2.1 the other day and popped my old SIM card in it. The day after I bought the phone, it started beeping at me at regular intervals because it was sending me lots of text messages telling me all my credit was gone. I topped it up with € 10 wondering why I suddenly had no more credit - although there was not much in the first place. To be on the safe side, I turned off a couple of running applications and checked the credit again at the end of the day to find that it had not been reduced.

That night I turned the phone off, but when I turned it back on again, I noticed that the credit was steadily going down. This time I can't seem to stop it. I also discovered that all the services/applications I previously turned off were now running again after the phone had been rebooted.

I can't say I understand the technology (yet), but I assume something in/on the phone is trying to call home (via the SIM card?). But what? I have now also turned off the WiFi and the location finding settings, but the credit is still going down.

The applications I have installed so far ar Market, Google Navigator, and My Tracks.