MTN doesn't care about their clients

They blocked my phone because I used another service provider's sim card, now they sending me to that service provider for help. I pay R880 every month for this Phone with MTN. I'm now paying for a phone that I can't even use. I went to the store (Cresta) Thabang told me they can't help me. I called their client care line, they don't know why I wasn't assisted at the store. Their Twitter page promised to investigate within 73 hours. Now they are ignoring me when I remind them about 73 hours.