pathetic service from retentions

I sent a mail to retentions to cancell a contract i had with MTN in early march with the understanding the last payment would be at the end of March as 30 days cancellation would be for this period. i spoke to a retentions agent on the 07/03 confirmed the cancelation. i was debited for March and now i get a bill for April as well. when querying with the Retentions they confirmed that cancelation was not done, so now another month i must eb debited for them not doing their job? they say i need to take it up with customer care and when i get to customer care they tell me i must take it up with retentions. absolute nonsense service and i should not be charged another month for a service that should have been cancelled and is not in use