Get a good metal whistle, not one of the plastic ones. You can tell if the call is coming from one of those annoying call centers if you don't hear anything at first, then when you say Hello, the call clicks in. They're voice activated. As soon as someone speaks, blow the whistle as hard as you can. I do it every time, and they rarely call back.
It's VERY hard to block these calls, because they have countless numbers they can call from. Their phones can be programmed to look like any number, any state. Tell your mother to never think any call of this nature is legit, and give her a loud metal whistle to blow in their ear. The # of calls I get have been greatly reduced using this method!
Other ways to get even....as a male, I have fun with this one. Raise your voice pitch just a little when you speak. "How are you today, big boy/girl?". "You sound delicious"...."Do you like handcuffs?"...."I've really been a bad boy! I think I need a spanking".....just keep going with anything you can think of. Sooner or later the people making the calls will demand their boss take your number out of the computer.