if they werent aware that you were trying to cancel the contract then it is 'fair' that they cut your phone off and continued to charge you, they will also take it to court if you cannot prove you cancelled it.
Write them a letter immediately, send it by recorded delivery, and state that you have been trying to cancel the contract since ?? and listing all the times you have called them. Even if they only keep record of calls for 2 months they will have details of your calls to same effect in December and November. Ensure that it is cancelled with immediate effect and then follow this up with a call to them a week later to confirm they have looked into the matter.