Telkom calls to me every day, why on earth?

I am receiving daily calls from Telkom. The number starts with *** or similar, the last digits differ. The phone disconnects when you answer. I mentioned this to one of my friends and she says this happens to her as well. Also numerous other people has reported this on the internet. I have blocked the number/s but the phone still rings and disturbs me while working. This is an invasion of privacy, I am not even a Telkom customer and have not given them permission to call me. What can I do to end this? Are they spying on people, what on earth is happening here? How widespread is this?