Telkom *****

My modem SIM number seem to be duplicated or hacked by another user as I’ve received a bill with calls I’ve never made to numbers I don’t know and I called to get clarity and no one could tell me what could have transpired. I’ve asked them to block the SIM card in the mean time while we not sure what going on. But the calls keep on happening and the bill is coming to me as we speak. What’s the next step from here as I didn’t get clarity