Deduction of different amounts every month

Hi I upgraded my contract in may 2015 and took out an extra contract which makes it two device all in all the amount was to be R420.00 each month to my suprise the first deduction was R502 the next R629, the next R694, the next one R609 and so forth , when I went and require I was told that I should call their call centers which go unanswered for hours or even days .Now I was told that they will hand over our contract to the head office to be reviewed , even if it is that so why the deducted amount is not the same..I am certainly not happy with the mtn services we cant use their call center line when you go visit the branch they cannot help you they send you back to calling their line.Everytime you find different workers by the branch so you have ti start all over again explaining your complaints and you get different response , I am sick of it its been 5 months now but to no avail