Unfair billing from MTN and Nashua Mobile

When Nashua mobile ceased to exist the outstanding amount on my account was handed over to a collections company and I made arrangements for payment. Next thing I know MTN started sending me statements for monthly data that I was not receiving. When I called MTN to alert them that I am already paying for the account with another firm they informed me that the account was handed over to them by Nashua and it reflected that I was receiving data monthly which I have never received. I have tried to contact Nashua mobile numerous times and it seems as though their lines are no longer working. Now I am getting threatening smses about further action on my account if I do not pay R3 300 immediately. Who is responsible for the incompetence in this scenario? Why should I be paying two companies for one account? I deserve answers and the least MTN could have done is to investigate the matter once I had alerted them instead of just continuing to bill monthly. I am infuriated that two prestigious companies such as MTN and Nashua can victimize a small Joe such as myself and then I have to incur costs for something I have never received!!!