Termination of contract after letter

I have received an invoice(13th May 2015) after requesting an immediate cancellation/termination of contract on the 30th April 2015. MTN acting *********ly and doing business as usual , ignoring my requests. Just for the sake of it or is it that they just do not care. I might have a different understanding of immediate to the one of MTN.<br> <br> I received threatening letters from MTN after missing an instalment, last being on the 29th April 2015. I then took up the suggestion on the termination of the contract and paid the amounts stated in the letter on the 30th April 2015. Now MTN seems to have forgotten or are unwilling to terminate the contract as per their suggestion/threat and my acceptance and compliance. I find this behaviour unacceptable. What was the point of sending such if MTN is not willing to follow through? I have also requested that no additional airtime be loaded on the account while they take their own sweet time.<br> <br> What is the matter with MTN? Cannot even get through to the call centre. Why?