In such circumstances, it will be better to have a talk with the senior dealing officer. Two months back I also received double amounted bill for my sky TV and I was 100% sure that it is not correct. So I have a talk with their dealing officer and explained them the whole situation. They checked my record and it was the mistake from their side. They sent me the rectified bill which I paid and my problem was over. In your situation if the calls were made from your phone then you have to pay the bill. As they have nothing to do with your health etc. So the best option is to co-operate with them.