Their fraud dept should sort this out. They will check where as well as when the calls were made. In all likelihood they will drop the charges, but bar the sim. They are unlikely to take you to court to recover the debt, as courts are more inclined to find in favour of customers who refute the charges than in favour of the mobile provider. It is most likely that your sim has been 'cloned', and the calls made to premium rate lines set up abroad by individuals experienced at conning money from mobile providers. In the worst case, if you are taken to court, it is not incumbent upon you to prove you didn't make the calles, rather that the mobile provider proves that you DID.
(I used to work for the largest mobile provider in the UK, in the fraud dept. We investigated hundreds of disputed bills each week, and in two years we took fewer than ten people to court)