If a device is used abroad, it's possible to rack up some hefty charges before anything gets back to the home network and rings alarm bells.
At best, you'll probably only be able to negotiate a goodwill reduction, I fear. I'd also speak to your home insurer and see if you have any cover with them.
Edited to add: Sorry, revisiting this, it's far too terse, but those are the facts as I see them. This is a horrible situation and a difficult one to resolve. The solution, for the future, is to always put a lock code on your SIM, but how many of us actually do that? - and especially when we're nor abroad in the first place. It's been suggested that people should only be liable for fraudulent use up to a certain limit, and the position is often compared to that with banks. There's an important difference, though: your bank card always has a PIN on it and you can be pretty sure that a limit on phone use would only apply if there were PINs (both device and SIM) in place at the time.
Anyway, I hope all this works out as well as possible for you. I'll flag this thread as needing attention as whatever process is going to take place needs to be started as soon as possible.
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