Keeps on calling even after I'm a supposed client

Just a few weeks ago a call centre called me, stating that they're from the unlimited heli rescue plus. They got my number from my sister (which I gave them as a number) stating that they can give me this whole package. I tried to tell the guy I'm already on their system but I didn't have my card with me to confirm, eventually he got the message that I already have their plan. But now they started calling me again, which makes me doubt the vadality of their product. I want to cancel my membership but don't know how.