you are probably rude to them in the first place. I mean it is understanable that you are annoyed with this kind of calls. I am not happy about them either. Anyway this is not the point.

Anyway, most probably they got your number from contacts that people sell. There are people that sell contacts for a price. And how these people got the contacts? From credit cards or bank employees who sell them for a fee. Things is, this kind of things cannot be avoided unless you don't give anybody your contact details.