CellC inflated December bill

My CellC bill for November and December was highly inflated - it listed a couple of numbers which I know for certain I had not dialed. The number that I apparently dialed the most on the Bill, I do not know, I don't have it on my telephone book and all they say is that the number must have been dialed from my phone and I will remain liable to pay the account. I am 100% sure that I would have remembered calling the same number 76 times in 1 month. I tried calling the number several times in the last week but it does not ring saying \not available at the moment". Sounds like the number does not exist. Also noted some other numbers on my bill which I know I did not dial over the December period."""