Billing incorrect

I made two phone calls on the 21/08/2015 time 10:00 and the second one on the 01/09/2015 time 15:45 as needed to unsuspend my account which i have to do every month i was then advised by one of the agents that my megabytes which i didn't utilize in the previous month will be carried over to September but now i was charged 1750 because i went over the 500megabytes that are allocated to me so i need to know why am i charged that as they said i will get a 1 gig on each sim. Why am i charged for the 500 megabytes that's i didn't utilized from the previous month please note i was advised by ur agent and you are welcome to pull up both those calls and listen to them.<br> i would appreciate this matter being sorted out as soon as possible. Thank you