Poor handeling of Nashua Mobile customers by MTN

I have been a MTN customer for close on 18 years. I even kept my contract running when I lived abroad for the last 4 years. This meant I paid but used no services or upgrades etc. I am now back in South Africa and using my existing contract which was through Nashua Mobile but with MTN. My phone was cut within the 1st week I was back. MTN said I had exceeded my credit limit - I have never set a limit or been made aware of a limit. They reinstated my phone after I complained and said it was sorted, this has now happened 6 times in 6 weeks. Today at MTN HO they said I have to prove my credit worthiness because they don't have my history on there system?? Is this my problem? One of the staff disclosed to me that this is a major issue with them taking over Nashua Mobile customers - they don't have the history. I have been a loyal customer for years and I have never been treated so badly, its disgusting. I want to close my contract and they want to charge me for the phone - even though for 4 years I never claimed a new phone until a year ago. I now have no working phone and a sick mother at home. Nice way to treat a loyal customer