Altech Autopage Breaking an Agreement

I contacted the up-grades department a few days ago and spoke to a Shadrack Vencatsamy in connection with my cell up-grade. He came back to me saying that I must pay an amount of R490.67 before he could do the up-grade because my account was 15 days in the arrears. I then explained to him that I am only supposed to pay the end of the month like I was doing since my contract was still with Nashua Mobile. He then said to me that his system says I am a cash payer and therefore my payment date was the 15th of every month. My average monthly account was R500 which I sorted out with them a few months ago and now suddenly it was R500 in arrears for half a month. I got the feeling that they want to get an extra R500 out of me before they do the up-grade. Not to mention that my average account with Nashua Mobile was R400 and when Altech Autopage took them over the average amount suddenly went up by R100. I must mention that I don`t use my cell phone a lot and it use to be more or less the same amount every month. I hereby want to warn the public to BE AWARE OF THIS, BECAUSE IF THEY TAKE R100 FROM EVERYBODY, THEY MAKE MILLIONS OF RANDS FOR NOTHING ! They didn`t want to send me a statement !