Vodacom cell phone contract

I have a contract with vodacom so when I lost my job I informed them to freeze my pack of airtime because it was not possible to continue paying it I could only affort to pay for the phone. They didn't so they kept send the monthly allocated airtime & I didn't pay for 2 months they disconnected me. They still charging me interest on the debt while the number is not working. I don't understand why they could continues charhing me interest while I cant even pay the principal debt.