My R499 contract keeps increasing every month

I signed up for a 24 month contract deal for R499 p/m including 200sms, 100 Anytime minutes and 300MB with a Samsung S5 in October 2014. For the last 3 months the amount has increased, I understood that there was going to be an increase on all contracts, but I thought this was once off and the new monthly amount would remain the same. The amount I pay varies now from month to month with the last one being R609.08 from R511 the previous month and around +-R530 the month before that. I tried calling their call centre to get an explanation, but couldn't get through at all after trying countless times. I sent emails and no response!! MTN is ripping me off as I still get the same package every month and I am on a Top-up contract so when it runs out I buy airtime and data bundles.