I bought a SIM card for 29.99 euros from Free mobile, offering unlimited data for 30 days in France. I used my credit card and they didn't take down any of my personal details. I used the address of the hostel I was staying at and probably entered my email address. I also used my nickname instead of real name because I figured its not even a mobile plan.

Now I traveled to Barcelona and used the roaming Internet (a priori included in the plan) a bit and they said I owe them more than 40 euros! Is it possible for them to charge me for it? I mean I thought it was a prepaid card and was supposed to stop when you finish using the credit! I bought the SIM from the machine in the store.