My contract

I’m on contract with Mtn and everytime I get my monthly minutes, sms and data they take what is left and give me new ones something I never signed up for why can’t they add on whatever balances they find on my phone and not take because I pay for it and it was never explained to me like last month I got my monthly contract deal and they promised to give me free 10GB on top of the one I suppose to get but when I got that my contract 10GB disappeared it doesn’t make any sense to me because at the end of the month I pay the full amount i would like to get that back and they should always add on and not take they shouldn’t limit me in how I use what I have paid for the number I’m reporting for is ********** 986 if they fail to fix this I’m going to terminate this contract immediately! I’m fed up.