Unhappy!!

This company keep harassing with phone calls saying I am unknowingly paying to mush interest and they could assist me in reducing the interest I pay. They keep calling even after I have said that I have no interest in their services. The moment I ask where they got my details and why they are calling me they eater put the phone down or say they will need to get back to me on this matter but never do so. It seems dubious at best. I need them to remove my details from their system and rereign from calling me.