INCORRECT BILLING

I was originally with Nashua Mobile but they closed their doors and now my contract is with directly with MTN. The last two months I've been overcharged by R49.80. I signed a 24-month contract for R200.00 and not R249.80. I've been trying for more than a month to get MTN to fix this. Very bad service and I don't think I will renew my contract later this year when it comes to an end. Never had this problem with Nashua Mobile. I was also told on the last mail I received that I can track my query on the website but when I put in the ref no, it shows no results found.