Mtn Bad customer service and Doing SIM lock on phone unnecessary

I regret taking a contract with Mtn ,I have been to their 2 branches in Cresta and Springs and I am not getting this SIM lock on phone resolved I have paid off their money and no longer have contract with them but they give you a phone with sim locked When you call service center they refer you to branch where they tell you that it's not possible How do I change phone that is sim locked? I don't owe them money which I expect myself to have full control over my phone and my advice to everyone everyone is that please don't take their contract if they don't clarify their things That's a horrible service I got If this is resolved I won't be having your SIM card even in future