Poor customer service

I had a device with MTN under contract. I always used a Vodacom sim card in the device, however my phone just suddenly one day lost signal and MTN couldn't assist me. Another service provider advised me that my device is blocked and can be used in another country outside of South Africa. When I took my device to the iStore, I was told that my phone isn't on the list for phones that have been blacklisted. I don't understand why MTN couldn't help me to resolve this, I had to buy a new phone whilst my phone is in perfectly good working order, it just cannot connect to any networks.