High billing

I phoned Vodacom about 2 months ago and asked why my cellphone account was so high and ask them to limit the 2 tablets to the amount of R200 each per month. I was assured that everything is in place. Still after receiving another high account I phoned them again on the 1 of Sept 15 asking the same question and request and was again assured that everything is in place. I received another account yesterday for the amount of R5091 and phoned them again and was informed that there is other factors for the cause of this high amount. Why was I not informed of this when making the first call. I gave them an instruction in my first call to put a limit of R200 each. Why did they not give me professional advise and what is Vodacom going to do to solve this problem? Thank you for your assistance.