Excessive billing&incompetence!!!

I am on contract&I upgraded from a blackberry to a Nokia Lumia.I took out a smart M contract as I was told that by doing this,my monthly bill would be no more than R350&if I was cautious with my data use,phone calls&smses,it would be much less.I never phone unless after 8pm where I receive free minutes,I never exceed my free smses,&I NEVER download to avoid exceeding my available monthly data.I have been receiving bills for +-R600 EVERY MONTH FOR OVER A YEAR!I've been in 3 times to query it,made numerous calls to the help centre,&all I get every time is a \oh