How does a blocked number be able to continuously call

I Receive several unknown calls and I've blocked them, but they continue to call back every single day. There is one caller from Consumer Advocacy - claiming to be able to lower my interest rates on my credit cards. I've told them I don't own credit cards and they hang up before I can request to be taken from their list. In the past I've pressed the corresponding option to be deleted but that doesn't help either. So what's the point in blocking them if they are able to continue this harassment?