Telkom billing customers for incoming calls it's a ****

Their network coverage is always the biggest problem as if that's not enough now they are ******** airtime out me,I had R16.00 airtime, I then received a call just after checking my balance after the call I wanted to buy data then boom I have insufficient funds,,HOW , I did not make a call they called me and it's not for the first time where I would be billed for an incoming call, always after I have dropped the phone I would receive SMS notification stating that I have less than R5.00