How can I stop collection agencies and other individuals who are mistakenly calling my grandmother?

They insist they are speaking to "Heather ____" although her name is Helen and she is elderly. She has had her phone number since late November. Half the time it is an operator and it just keeps saying stay on the line if you ARE this person. The other half, it is a live call from multiple angry people who insist she is Heather. Her number was put on the don't call list 2 months ago, but this hasn't helped.