No service from customer service

I phoned Cell C wanting to upgrade my contract as my phone is used for my work from this year and to avoid skyrocketing bills i want to change my contract. I phoned customer service four times and each time the person promised to call me back within the day because they needed to do a credit check. Each time the person said they will find out what is going on and call me back by the end of the day. It has been a whole month now that i have waited and still have no response. I am furious as my phone bill is sky high and they do not want to help me to change my contract!