Mrpmobile cellphone contract debit order amount not the same as the agreed it differs every month

Hellpeter kindly assist me on this Mrpmbile i took a cell phone contract with them it will be three months with them no when i took the contract they told me its R159.00 per month but they dont deduct that amount from my bank account 2 months back n last month they deducted R252 ref Mrpmobile Invoice *** i tried to call them n their lines its just ringing none stop and sad part is early last month ii called i asked them why do they deduct more that what we agreed on n the lady i spoke to said it will be stopped with immediate effect only to find out end of August was even worse R252.00 Please help or advice. Thank you