applying contract which i did not sign for

I was a Nashua Mobile client and when they closed my contract went straight to MTN. They app**** their own charges for my contract and not the contract that I signed for at Nashua. I have contacted the call centre several times and they said they will do investigations and request the contract from Nashua, but after 3 months nothing has been done. This is know the forth month that I am over paying for my cellphone account. At this point MTN is owing me more than money then what my contract is per month. I was a client of Nashua for more than 8 years and never had any problems with client service.