Cell C
Charges when payment is up to date

I have a complaint against CELL C, I went to them to change my banking details and was told to come another day, reason being they said they was busy, so I've been doing deposits, now cell c sends me a message saying that I'm in arrears because they are charging me a penalty fee, I am up to date with my payments, so I don't see why I should b charged this extra fee, I'm very disappointed with cell c, I have five contracts with them, thank you