Nuisance calls from Cell C and I'm not even a customer

I don't have a Cell C contract but receive a daily nuisance call on my landline from them, always asking for a Linda Mthambo. This person doesn't live here and I have asked the agents to stop calling and to update their records - the moment I start to complain and ask them to correct their records, most times the agent hangs up without another word. It has happened very day for the past month. If this does not stop, I am reporting Cell C to the ombudsman for nuisance communication.