How does billing work?

Can someone please explain VM billing? I usually pay when I get a bill because my income is paid on different dates. On December 7th I paid £207 which as far as I was concerned, brought me up to date. I waited to get the next bill so I knew how much to pay. I guessed it might be the start of January and then I didn't think about it until a bill turned for £207 just yesterday. I am so confused and always have been with the billing system. That is a lot of money in one go. I'm not a numbers person at all so I am unable to work this out. To be honest, VM have disillusioned me so much that I have got to the stage where I can't be bothered to try to work it out. But some kind person on here might be able to understand and explain to me once and for all. Thanks in advance.