Incorrect billing and failure to cancel

I bought an existing business who had 8ta contract. I renewed and upgraded contract with all my new details on the contract. With 4x Samsung s4 mini's. 3 of the phones never worked as the locked sims numbers belonged to other people. 8ta could not change the sims or fix this problem. So I paid for 3 phones for 2 years I have never been able to use. (Trust me I tried sorting this problem out to no avail). I was also charged for Blackberry BIS. (On Samsung phones). Although my name on contract, they refused to speak to me as they wanted to liaise only with previous owner of company (Even though I changed everything and contract in MY name). I gave more than 1 month notice cancelling contract after the 2yrs (Being feb 2016) I received confirmation they have received my cancellation request, but have just received an invoice for March for month to month billing. I am so frustrated. Nobody in the WHOLE company can assist me, its like extracting hens teeth. This is **************** and I want a refund. Worst customer service I have ever had because it is NON EXISTANT!