Harrassment on paid account

Being harrassed for a Cell C account which was settled directly to Cell C, advised Nimble to discuss this with their client Cell C as the correct channels were not followed and I did pay Cell C. 6 months not a word from them and all of a sudden I get a offer for discount on the already paid account? I think I need to refer this to National credit regulator as Nimble is not attending to my query raised and proof of account being paid