Don't use MTN

Morning, I need urgent advice on what I can do please. About six months back we received a call from a mtn agent asking to change our phone contracts package without any extra costs. About a month later we received a sms stating we owe R2000 additionally. Went to pay it and ask that they stop ALL changes that were made. The lady in the shop said it's fixed. December we received another sms stating we owe R6700 . After we went to the store & phoned head office and they assured us everything is fixed. We payed it. Without arguing or questioning the amount,February same thing received a sms stating we owe R8000 . If it's not payed they switch off your signal . No one from the mtn shops or head office has been able to assist after numerous attempts & are able to tell us why the amount is so high we are on wifi at work and home so it's almost impossible for the phone account to be so high for data. It seems like every three monts the account just gets higher and higher.