germano is absolutely correct - this may be a big part of the issue. (and the fact that phone rates are pretty high even for messages).
My family has been plagued with this problem. And it can be a bitch to get the services off your number. Particularly if you bought a SIM off the street, not at an actual representative of one of the carriers. From strange calls you get looking for bread, I wonder if maybe you were resold a chip from a stolen phone that wasn't reported to the carrier?
You may not even realize you have accepted these services. I had a phone (don't actually know if it was a problem on my phone itself) that the message would display on my screen and would have no "close" or "reject" (whatever) button to get rid of it, just an "Ok". I can remember once or twice being in a hurry and hitting the "OK" thinking it was a regular SMS message, but it was one of these services, sent by a special Personal type of message, and by clicking on OK instead of shutting the window, I accepted the service.
Also, do you ever use "borrowed" time when you run out? I don't remember how bad that was (get a prepaid plan if you have a DNI!), but it seemed they used to take a good bit of "interest" (i.e., more than you borrowed) out of the next time you charged your line's balance.
