'MTN and Mondo deducting money for a cellphone contract that was never activated

I bought a contract phone via Mondo last year April i returned it the next day after receiving it. In August MTN started deducting money for the phone till now. I have been calling monthly to stop this but i couldn't stop them. Last month i was advised to stop the contract which i did then they sent me a bill of the remaining contract term to pay. i called still no help so im thinking they are going to try and deduct this money again this monthend since they couldn't deduct it last month. When i call mondo they say mtn is the one deducting when i call mtn they say mondo must fix billing. so i don''t know what to do anymore.