I am sorry to say that if you read your contract, you are responsible for paying all the call charges run up on your phone. How can it be Vodaphone's fault that you lost your phone? I also think your contract will say that you remain liable for fraudulent activity until it is reported to them. Whilst I am sympathetic to your problem, I fear that there is little you can do other than negotiate with Vodafone, but in reality, you may well end up with a CCJ against you if they refuse (as a gesture of good will, since they have no obligation to do so) to reduce your bill.