Billing Query

In the month of Dec and Jan I was billed for x2 3G data on one of my contract phones, according to the call centre a promotional SMS was sent and the user opted in. Since Jan I have been calling MTN to query this as I did NOT authorise this. I then requested that MTN provide me with PROOF that the user opted in and PROOF that I the account holder has authorised this additional amount to be debited from my account. 2 weeks ago I was told that the query was sent to Marketing, today I'm told again the query must be sent to Marketing. I'm not sure this query will ever reach Marketing? What puzzles me is that when I contact MTN regarding any of my contract phones I have to provide my MTN PIN and authenticate myself as the account holder regardless of the query, however when promotional SMS's are sent NO authentication is need from the account holder???? By law information needs to be kept for 5 years, all I'm asking for is PROOF the user opted in on YOUR promotional SMS and PROOF that I the account holder with MTN has authorised this!!!!!<br>