Premium-rate service drained my Virgin mobile balance - but I hadn't signed up

I have lost about £20 to people who had no right to take my money I put £20 on my pay-as-you-go Virgin mobile account. When I checked the balance a couple of weeks later I found I only had £1.28 credit, which was a big surprise as I don't use it very much. I then noticed a text message informing me I could unsubscribe to a firm I had never heard of, and another saying that I didn't have enough funds for the service to continue. I contacted Virgin and got through to a customer service person. ...