Double billing since inception and no response

Nashua Mobile was taken over by MTN last year.<br> <br> since this has happened I have been billed twice every month for my 500 sms's which should cost me only R100.<br> <br> They do not answer their emails nor their call centre calls.<br> <br> If I return the debit order I will be without a cellphone which I can not afford.<br> <br> All I want is a refund for the double fees and a confirmation that the 500 sms will only be charged once a month going forward.<br> <br> Please can you help?<br>