3 weeks since reported problem no solution yet

Im not able to receive calls 99% of the day. Calls are either dropped without ringing, diverted to voice mail or answered by an unknown person. I've reported this numerous times. Cases were logged at the technical departement, three sim swapps were done - for which I had to pay, and still the problem presists. Everytime the people at the call centre and technical department promised that the problem is resolved just to find that the same problem re-appears after a few minutes. Ive tried all sim cards in 4 diffrent phones, so the problem is not with my phone. I dont know where to seek assistance to solve the problem. Im loosing business as a result of CellC unability to resolve my problem on the line I've used for more than 15yrs without these kind of problem.