I DON'T HAVE A VODACOM CONTRACT

I don't have an account with Vodacom, I didn't sign a contract with them, but I am constantly getting call from them and when I tell them I don't have an account they just drop the call on me. I don't even know how they got my number from but know it's my problem and they want me to go to the police station before they can do their jobs. Why didn't you get my number and phone me before you just give someone a phone and contract.