Bad service

I have a contract with them and struggling to cancel it. I have paid for the last 24 months and get airtime and data whenever they feel like it. I am trying to cancel the contract but getting no joy out of it. I am told I owe April payment and I give supp**** them with POP still they making excuses that I have to wait for amount to be allocated. How an internal issue is becoming a customer problem? I have been emailing and getting no response. Calling their office number for some reason no one picks up the phone. I really need this contract closed.