Vodacom Harassment.

I am being harassed by Vodacom. I do not answer unknown numbers. I call the number back to see who it was, but in this case the range of numbers (+27 10 495 7???) do not ring or answer. The caller does not leave a voice message. They call me on average 3 times per day and once on Saturday every day for over a month. This is working out to be over 100 calls. If I do not answer the first ten calls, what makes them think that I will answer after 100? On one occasion I was outside and my wife answered. The caller said that she was from Vodacom and agreed to call back after 30 minutes. She did not and I received none of these calls for a few days. That shows how honest and reliable they are - they call me three times a day, but not when they promise to do so. This is not bordering on harassment - this IS harassment. And the funny thing is, is that I have just upgraded my contract, so what are they trying to sell me?