Contract dispute

So I've had a cellphone contract with mtn for years, I went for an upgrade at a mtn store in westgate mall in September last year, I shifted the contract from my sister to my name. The package was for two phones. Only one phone was shifted, I'm paying for the whole package and yet the other phone is blocked and billed separately. I've visited the store a lot of times trying to sort it out with no luck. I'm considering terminating my contract.