Bad service and communication

On the 26th of March 2016 I cancelled my contract with glocell I received confirmation that on the 30th April 2016 my number would go over to a prepaid number. On the 1st May 2016 I received airtime, I sent emails and tried to phone to find out why I got no response from the emails and the call centre kept cutting me off. I refuse to be held liable for payment for airtime that I did not ask for. I also think it is in bad taste that the staff can't do their jobs. I wish that you can sort this matter out please