Incorrect billing

I need someone to explain to me why my Cell C bill is not the same amount each month. On my December invoice I was billed R322. 09. January came and I had a debit order of R493. 12 and I did not even receive a statement or invoice from cell c this month! This is unbelievable! They charged me R 171.03 this month! This needs to be explained to me as I am not keen on Network providers that does this to their clients! Thank God I have dicided to cancel my contracts within the next few months with Cell C. Unbelievable.