Mtn changes a confirmed emailed agreement without my consent

I communicated with Mtn One Touch Mobile and Retensions with the SPECIFIC agreement that my cancellation of my 12 month contract FTU home internet happens on the LAST day of my contract, 11 April. I received EMAIL confirmation FROM Mtn at 12 noon on 01 April (with a screenshot of the billing) that CONFIRMS the cancellation date as 11 April at NO (R0.00) cost to me. BUT. After 10PM (10 hours later) I receive an SMS stating my cancellation date has been changed to 01 April and that I would be CHARGED accordingly. This is NOT the arrangement I have Email proof of. This is NOT ethical. This CANNOT incur a charge from my account that I was ASSURED in WRITING, that I would NOT be charged. I will view this as theft.