penalty fee?????

Sept 1 2014 I upgraded my phone from my call 350 to my call 100 With Nashua mobile. They told me because I upgraded to a lower contract I will pay R240 penalty on my handset until 1st Des, then only my new contract will start. 1st Dec come and go, nothing was changed, so I went to MTN with my contract. After 2 months they change it, but lock my phone because my bill was R2800 + high, because MTN decided to ask me penalty fee of over R2000 because I dare to lower my contract AFTER my 2 year contract with NM has expired. It took me 2 day to reconnected my phone. But they don't want to help me sort out my bill. They not even interested in listening to my explanation.