Poor customer service

I called Discovery vitality to cancel my membership no was told that all the agents were busy and that they would attend to my call within 10 days. Firstly it is ridiculous that it takes them 10 days to call a client back, I understand that I am not the only customer but even cellphone networks are more efficient than that. Secondly, more than 10 days later I have still not received a call back and this is the second time they have promised to call back and have not done so