Contract Cancellation

I am greatly disappointed by MTN, my contract with them ends this month as they have deducted the last installment. I called in early in the month to report that I will not be upgrading or continuing with my contract and asked that they cancel and was advised to only call after my last installment is deducted. I then called back on the 29th a day after they had deducted to ask that my contract is cancelled and requested for a cancellation letter and was told that my contract can only be cancelled after 30 days. This is a huge inconvenience to me as I need to use a cancellation letter to prove that MTN will no longer be part of my obligations and the lady I spoke to specifically said to me they don’t send letters just an sms to show that a cancellation has been requested. I further wrote an email and have not received any response.