unbelievable conduct

I changed myprepaid landline to fixed adsl on 14 th Dec 2014. But on 28 th I could not connect to the Internet via m Web. I then decided to cancel my order. I had already received a bill for that month. I then cancelled my line. Iend of January. February.and March I still received bills. Now I owe 705 rdespite the fact thatmy phone had been changed to prepaid again. Please help me as all my visits to the nearest bluff shop has yielded no joy. I