Incompetence

I have requested mtn to change my debit order's banking details 4th March, as i want to close my Fnb bank account,. they advised me to phone a number, which does not work from a landline and i am not going to phone of my cell phone as the banking details pertains to a 3G. Eventually i received a mail asking for all kinds of security questions. Which i answered. on the 12th March they asked me for a contact number which i sent to them on the same day. on 7/7 i emailed to say that i haven't received any phone call to change the banking details. the debit order has bounced because of lack of interest from MTN or incompetence as i have sent another email asking to be phoned - and MTN has read the mail because i received a read receipt!!! BUT no phone call.Who has to pay that R100 bank fees?????