Incorrect billing account query

Have had numerous calls made to my cell phone. Terrible experience, I have called the RCS/ Nimble/ Edgars call center on numerous occasions /been in to Edgars regarding my incorrect billing, they are charging me interest every month on outstanding bill which I paid despite not having purchase anything from them for quite a while. The bill was R30.00 and has now increased to R300.00. When you call them, they keep you on hold and then they say they will escalate the matter but no one ever calls. They are now calling me at my place of work which is disturbing and interrupting my concentration.