AIrtime keeps on being depleted on Prepaid card with no calls being made.

I have a prepaid number for which I keep buying airtime which keeps on disappearing. The first time I bought R 40.00 airtime two weeks ago on a Friday night. I recived R40 plus some free airtime for using the App which came to R 66.00 airtime. Despite my not having made any calls and only having sent one SMS By Monday I only had R4 .000 airtime left. Don't tell me it is background App because I have fiber at home to which I am connected and I have data on my phone. Where has my airtime gone to? This weeked same thing I but R12.00 airtime to send an SMS because I do not trust them I Get R 16.00 because of the App. by Monday I only have R 1.40 airtime left not a single call made?? What is going to be done about this we keep on getting our airtime ******. This never happened when I was on contract.