I once got phone calls on my cell phone about some lottery I was taking part and if I wanted to renew for three months - only I had never heard of this before and had never signed anything or over the phone agreed on taking part. I finally decided not to answer these calls any more (it showed the number) which resulted in them calling me up to 20 (!) times a day. So I checked my phone out and found a way how you can automatically reject calls without the phone even ringing, which I did.