Incorrect Billing

I am currently experiencing an issue with my second instalment which has to be R625 per month. At first I paid as you pay for upfront, I paid a little bit extra because of the usage of the first month of which I started mid Dec, so my January account payment was R800 something. And when I enquired the lady confirmed that you paid for mid December and upfront for the whole of January. Now they want R910 for February, how did we get there from an agreement of monthly instalment of R625 to R910? And it can’t be top ups on the sims, because I asked that I don’t want any top ups on my SIM cards because currently there’s only one working. I can’t even review the receipt on the app.