Telkom Mobile's refusal to cancel my contract

I informed Telkom on 8 February, via email, of my intent to cancel my Telkom Mobile contract, with effect from 1 April. Received no confirmation of receipt from them, and have not phoned in twice. On each occasion they claim not to have received the notice of cancellation, with a promise that the issue will be escalated, and that I will be contacted. Needless to say, I have not been contacted by anyone....And have just received an invoice for April. So it seems that Telkom simply refuses to cancel my contract.