Different billing amounts every month

I previously was with Nashua Mobile, who closed down and automatically was moved to MTN. Ever since I was moved there, my bill differs every single month, even though my contracts taken out for 3 cell phones stay the same and should be that amount every single month. Try calling the call centre only to be told they don't understand why this is happening and log a call, but nobody ever comes back to me to say what the problem is. Myself and both my childrens usage is locked and permanently set to \0"so we never go over the usage so that can't be what makes my account differ every single month!!!!! This is pathetic and when I am again due for an upgrade"