Incorrect Billing - Debit order

I went to Mtn to do my upgrade as normal, but when it came to debiting my account they still billed me according to my previous contract which was higher. I didn't't notice it until my wife pointed it out to me .When I queried it with Mtn they said I had to email them my signed contract with the new billing. I have done that and still they keep debiting my account incorrectly. I have tried to phone the customer call centre but there is no answer I have even emailed them. This is very poor service from a big company such as themselves. I have been waiting for this error to be fixed for almost 2 months now. They have been incorrectly debiting my account for more than 5 months now. When will I get my money back and my account rectified!