INCORRECT DEBIT ORDER, STRUGGLE TO GET REFUND

I am on 24 months contract. For 12 months my debit order remained the same, as I signed for, but from March 2015, MTN billed me every month for an extra 500MB. I already have one 500MB loaded with my contract, but the loaded an extra bundle and charged me for it without me requesting an extra bundle.<br> From March I phoned and e-mailed them to cancel and refund. I received one refund. The contact centre every time confirms that it is cancelled and give me a reference number, but every time when I receive my statement, the extra bundle is billed again. I requested my refund for end April's refund, but up to date have not received it. Every time I phone I request that the mistake be rectified before the debit order goes off at end of the month, but the incorrect amount is still deducted. I try to phone their contact centre but sometimes I am put on hold for 40 minutes without getting help. My e-mails do not get answered. I have been a client for 15 years already and this is the worst client service I received!