Overcharging and breaching contracts

I signed a contract with vodacom to pay R220 a month for a contract.

They are overcharging me and are charging me R299 instead of the amount signed for on the contract.

I have been to different branches and has called on numerous times to get it resolved.

No one has assisted as of yet.

They tried correcting this and sent me a new quotation form however does not have my signature on.

Please let someone from your CEO office call me ********** 936 **********

I would like to take legal actions

Thank you