Negligence by MTN employee could cost a client dearly

Recently experienced a small case crime whereby I lost my phone. Upon my arrived at MTN with a case number having reported the crime to the police the consultant told me not insured. Whipped out my contract summary and asked how so what’s the amount I’ve been paying for? Long and short of it is that I’ve overcharged for a device I didn’t get and the difference between the contract I should be paying with insurance almost equates to that amount, how do I solve this issue?