Upgraded to new phone but no delivery was made

I upgraded to new phone on the 31st of August this year and the upgrade was done on the MTN system and no phone was delivered to me. According to MTN staff the phone was released to the corrior company but the company return the phone back to MTN claiming that I am not reachable. I am continously paying for the new phone which I have not received. I need help because the MTN local staff said there's nothing they can do for me.