Paying for Nothing

I have taken out a MTN contract through Mondo in June 2021 and one of the two lines were wrongly issued. When I called Mondo in July they acknowledged their fault and promised to cancel the line. I have since then been calling Mondo and they keep telling me that the request has been sent to MTN to effect. My numerous calls have been for nothing, Mondo makes me regret ever trusting them to offer the service they promise on their adverts. I am fed up.